Auto merge of #23325 - emilio:gecko-sync, r=emilio

style: Sync changes from mozilla-central.

See each individual commit for details.

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53 files changed, 967 insertions, 874 deletions

diff --git a/components/layout/display_list/gradient.rs b/components/layout/display_list/gradient.rs
index db5208eadac..9b19ee48299 100644
--- a/components/layout/display_list/gradient.rs
+++ b/components/layout/display_list/gradient.rs
@@ -12,7 +12,7 @@ use style::values::computed::image::{EndingShape, LineDirection};
 use style::values::computed::{Angle, GradientItem, LengthPercentage, Percentage, Position};
 use style::values::generics::image::EndingShape as GenericEndingShape;
 use style::values::generics::image::GradientItem as GenericGradientItem;
-use style::values::generics::image::{Circle, Ellipse, ShapeExtent};
+use style::values::generics::image::{Circle, ColorStop, Ellipse, ShapeExtent};
 use style::values::specified::position::{X, Y};
 use webrender_api::{ExtendMode, Gradient, GradientBuilder, GradientStop, RadialGradient};
 
@@ -92,7 +92,14 @@ fn convert_gradient_stops(
     let mut stop_items = gradient_items
         .iter()
         .filter_map(|item| match *item {
-            GenericGradientItem::ColorStop(ref stop) => Some(*stop),
+            GenericGradientItem::SimpleColorStop(color) => Some(ColorStop {
+                color,
+                position: None,
+            }),
+            GenericGradientItem::ComplexColorStop { color, position } => Some(ColorStop {
+                color,
+                position: Some(position),
+            }),
             _ => None,
         })
         .collect::<Vec<_>>();
diff --git a/components/layout/fragment.rs b/components/layout/fragment.rs
index ff62cc79638..ccc3de5ec7e 100644
--- a/components/layout/fragment.rs
+++ b/components/layout/fragment.rs
@@ -61,8 +61,8 @@ use style::selector_parser::RestyleDamage;
 use style::servo::restyle_damage::ServoRestyleDamage;
 use style::str::char_is_whitespace;
 use style::values::computed::counters::ContentItem;
-use style::values::computed::{LengthPercentage, LengthPercentageOrAuto, Size};
-use style::values::generics::box_::{Perspective, VerticalAlign};
+use style::values::computed::{LengthPercentage, LengthPercentageOrAuto, Size, VerticalAlign};
+use style::values::generics::box_::{Perspective, VerticalAlignKeyword};
 use style::values::generics::transform;
 use style::Zero;
 use webrender_api::{self, LayoutTransform};
@@ -2397,47 +2397,49 @@ impl Fragment {
             // FIXME(#5624, pcwalton): This passes our current reftests but isn't the right thing
             // to do.
             match style.get_box().vertical_align {
-                VerticalAlign::Baseline => {},
-                VerticalAlign::Middle => {
-                    let font_metrics =
-                        with_thread_local_font_context(layout_context, |font_context| {
-                            text::font_metrics_for_style(font_context, self.style.clone_font())
-                        });
-                    offset += (content_inline_metrics.ascent -
-                        content_inline_metrics.space_below_baseline -
-                        font_metrics.x_height)
-                        .scale_by(0.5)
-                },
-                VerticalAlign::Sub => {
-                    offset += minimum_line_metrics
-                        .space_needed()
-                        .scale_by(FONT_SUBSCRIPT_OFFSET_RATIO)
-                },
-                VerticalAlign::Super => {
-                    offset -= minimum_line_metrics
-                        .space_needed()
-                        .scale_by(FONT_SUPERSCRIPT_OFFSET_RATIO)
-                },
-                VerticalAlign::TextTop => {
-                    offset = self.content_inline_metrics(layout_context).ascent -
-                        minimum_line_metrics.space_above_baseline
-                },
-                VerticalAlign::TextBottom => {
-                    offset = minimum_line_metrics.space_below_baseline -
-                        self.content_inline_metrics(layout_context)
-                            .space_below_baseline
-                },
-                VerticalAlign::Top => {
-                    if let Some(actual_line_metrics) = actual_line_metrics {
-                        offset =
-                            content_inline_metrics.ascent - actual_line_metrics.space_above_baseline
-                    }
-                },
-                VerticalAlign::Bottom => {
-                    if let Some(actual_line_metrics) = actual_line_metrics {
-                        offset = actual_line_metrics.space_below_baseline -
-                            content_inline_metrics.space_below_baseline
-                    }
+                VerticalAlign::Keyword(kw) => match kw {
+                    VerticalAlignKeyword::Baseline => {},
+                    VerticalAlignKeyword::Middle => {
+                        let font_metrics =
+                            with_thread_local_font_context(layout_context, |font_context| {
+                                text::font_metrics_for_style(font_context, self.style.clone_font())
+                            });
+                        offset += (content_inline_metrics.ascent -
+                            content_inline_metrics.space_below_baseline -
+                            font_metrics.x_height)
+                            .scale_by(0.5)
+                    },
+                    VerticalAlignKeyword::Sub => {
+                        offset += minimum_line_metrics
+                            .space_needed()
+                            .scale_by(FONT_SUBSCRIPT_OFFSET_RATIO)
+                    },
+                    VerticalAlignKeyword::Super => {
+                        offset -= minimum_line_metrics
+                            .space_needed()
+                            .scale_by(FONT_SUPERSCRIPT_OFFSET_RATIO)
+                    },
+                    VerticalAlignKeyword::TextTop => {
+                        offset = self.content_inline_metrics(layout_context).ascent -
+                            minimum_line_metrics.space_above_baseline
+                    },
+                    VerticalAlignKeyword::TextBottom => {
+                        offset = minimum_line_metrics.space_below_baseline -
+                            self.content_inline_metrics(layout_context)
+                                .space_below_baseline
+                    },
+                    VerticalAlignKeyword::Top => {
+                        if let Some(actual_line_metrics) = actual_line_metrics {
+                            offset = content_inline_metrics.ascent -
+                                actual_line_metrics.space_above_baseline
+                        }
+                    },
+                    VerticalAlignKeyword::Bottom => {
+                        if let Some(actual_line_metrics) = actual_line_metrics {
+                            offset = actual_line_metrics.space_below_baseline -
+                                content_inline_metrics.space_below_baseline
+                        }
+                    },
                 },
                 VerticalAlign::Length(ref lp) => {
                     offset -= lp.to_used_value(minimum_line_metrics.space_needed());
@@ -3087,15 +3089,22 @@ impl Fragment {
     /// Returns true if any of the inline styles associated with this fragment have
     /// `vertical-align` set to `top` or `bottom`.
     pub fn is_vertically_aligned_to_top_or_bottom(&self) -> bool {
-        match self.style.get_box().vertical_align {
-            VerticalAlign::Top | VerticalAlign::Bottom => return true,
-            _ => {},
+        fn is_top_or_bottom(v: &VerticalAlign) -> bool {
+            match *v {
+                VerticalAlign::Keyword(VerticalAlignKeyword::Top) |
+                VerticalAlign::Keyword(VerticalAlignKeyword::Bottom) => true,
+                _ => false,
+            }
+        }
+
+        if is_top_or_bottom(&self.style.get_box().vertical_align) {
+            return true;
         }
+
         if let Some(ref inline_context) = self.inline_context {
             for node in &inline_context.nodes {
-                match node.style.get_box().vertical_align {
-                    VerticalAlign::Top | VerticalAlign::Bottom => return true,
-                    _ => {},
+                if is_top_or_bottom(&node.style.get_box().vertical_align) {
+                    return true;
                 }
             }
         }
diff --git a/components/layout/inline.rs b/components/layout/inline.rs
index bef95a64f96..f4dacefc5ef 100644
--- a/components/layout/inline.rs
+++ b/components/layout/inline.rs
@@ -41,7 +41,7 @@ use style::logical_geometry::{LogicalRect, LogicalSize, WritingMode};
 use style::properties::ComputedValues;
 use style::servo::restyle_damage::ServoRestyleDamage;
 use style::values::computed::box_::VerticalAlign;
-use style::values::generics::box_::VerticalAlign as GenericVerticalAlign;
+use style::values::generics::box_::VerticalAlignKeyword;
 use style::values::specified::text::TextOverflowSide;
 use unicode_bidi as bidi;
 
@@ -1269,13 +1269,13 @@ impl InlineFlow {
         let mut largest_block_size_for_top_fragments = Au(0);
         let mut largest_block_size_for_bottom_fragments = Au(0);
 
-        // We use `VerticalAlign::Baseline` here because `vertical-align` must
+        // We use `VerticalAlign::baseline()` here because `vertical-align` must
         // not apply to the inside of inline blocks.
         update_line_metrics_for_fragment(
             &mut line_metrics,
             &inline_metrics,
             style.get_box().display,
-            GenericVerticalAlign::Baseline,
+            VerticalAlign::baseline(),
             &mut largest_block_size_for_top_fragments,
             &mut largest_block_size_for_bottom_fragments,
         );
@@ -1322,11 +1322,20 @@ impl InlineFlow {
             largest_block_size_for_top_fragments: &mut Au,
             largest_block_size_for_bottom_fragments: &mut Au,
         ) {
+            // FIXME(emilio): This should probably be handled.
+            let vertical_align_value = match vertical_align_value {
+                VerticalAlign::Keyword(kw) => kw,
+                VerticalAlign::Length(..) => {
+                    *line_metrics = line_metrics.new_metrics_for_fragment(inline_metrics);
+                    return;
+                },
+            };
+
             match (display_value, vertical_align_value) {
-                (Display::Inline, GenericVerticalAlign::Top) |
-                (Display::Block, GenericVerticalAlign::Top) |
-                (Display::InlineFlex, GenericVerticalAlign::Top) |
-                (Display::InlineBlock, GenericVerticalAlign::Top)
+                (Display::Inline, VerticalAlignKeyword::Top) |
+                (Display::Block, VerticalAlignKeyword::Top) |
+                (Display::InlineFlex, VerticalAlignKeyword::Top) |
+                (Display::InlineBlock, VerticalAlignKeyword::Top)
                     if inline_metrics.space_above_baseline >= Au(0) =>
                 {
                     *largest_block_size_for_top_fragments = max(
@@ -1334,10 +1343,10 @@ impl InlineFlow {
                         inline_metrics.space_above_baseline + inline_metrics.space_below_baseline,
                     )
                 },
-                (Display::Inline, GenericVerticalAlign::Bottom) |
-                (Display::Block, GenericVerticalAlign::Bottom) |
-                (Display::InlineFlex, GenericVerticalAlign::Bottom) |
-                (Display::InlineBlock, GenericVerticalAlign::Bottom)
+                (Display::Inline, VerticalAlignKeyword::Bottom) |
+                (Display::Block, VerticalAlignKeyword::Bottom) |
+                (Display::InlineFlex, VerticalAlignKeyword::Bottom) |
+                (Display::InlineBlock, VerticalAlignKeyword::Bottom)
                     if inline_metrics.space_below_baseline >= Au(0) =>
                 {
                     *largest_block_size_for_bottom_fragments = max(
diff --git a/components/layout/table_cell.rs b/components/layout/table_cell.rs
index 682a1c38ec4..ec79f9f4540 100644
--- a/components/layout/table_cell.rs
+++ b/components/layout/table_cell.rs
@@ -23,7 +23,7 @@ use style::logical_geometry::{LogicalMargin, LogicalRect, LogicalSize, WritingMo
 use style::properties::ComputedValues;
 use style::values::computed::length::Size;
 use style::values::computed::Color;
-use style::values::generics::box_::VerticalAlign;
+use style::values::generics::box_::{VerticalAlign, VerticalAlignKeyword};
 use style::values::specified::BorderStyle;
 
 #[allow(unsafe_code)]
@@ -138,11 +138,11 @@ impl TableCellFlow {
             self.block_flow.fragment.border_padding.block_start_end();
         let kids_self_gap = self_size - kids_size;
 
-        // This offset should also account for VerticalAlign::Baseline.
+        // This offset should also account for VerticalAlign::baseline.
         // Need max cell ascent from the first row of this cell.
         let offset = match self.block_flow.fragment.style().get_box().vertical_align {
-            VerticalAlign::Middle => kids_self_gap / 2,
-            VerticalAlign::Bottom => kids_self_gap,
+            VerticalAlign::Keyword(VerticalAlignKeyword::Middle) => kids_self_gap / 2,
+            VerticalAlign::Keyword(VerticalAlignKeyword::Bottom) => kids_self_gap,
             _ => Au(0),
         };
         if offset == Au(0) {
diff --git a/components/layout/text.rs b/components/layout/text.rs
index b26e531e670..199bd7e07f2 100644
--- a/components/layout/text.rs
+++ b/components/layout/text.rs
@@ -21,13 +21,13 @@ use std::collections::LinkedList;
 use std::mem;
 use std::sync::Arc;
 use style::computed_values::text_rendering::T as TextRendering;
-use style::computed_values::text_transform::T as TextTransform;
 use style::computed_values::white_space::T as WhiteSpace;
 use style::computed_values::word_break::T as WordBreak;
 use style::logical_geometry::{LogicalSize, WritingMode};
 use style::properties::style_structs::Font as FontStyleStruct;
 use style::properties::ComputedValues;
 use style::values::generics::text::LineHeight;
+use style::values::specified::text::{TextTransform, TextTransformCase};
 use unicode_bidi as bidi;
 use unicode_script::{get_script, Script};
 use xi_unicode::LineBreakLeafIter;
@@ -738,23 +738,23 @@ fn apply_style_transform_if_necessary(
     last_whitespace: bool,
     is_first_run: bool,
 ) {
-    match text_transform {
-        TextTransform::None => {},
-        TextTransform::Uppercase => {
+    match text_transform.case_ {
+        TextTransformCase::None => {},
+        TextTransformCase::Uppercase => {
             let original = string[first_character_position..].to_owned();
             string.truncate(first_character_position);
             for ch in original.chars().flat_map(|ch| ch.to_uppercase()) {
                 string.push(ch);
             }
         },
-        TextTransform::Lowercase => {
+        TextTransformCase::Lowercase => {
             let original = string[first_character_position..].to_owned();
             string.truncate(first_character_position);
             for ch in original.chars().flat_map(|ch| ch.to_lowercase()) {
                 string.push(ch);
             }
         },
-        TextTransform::Capitalize => {
+        TextTransformCase::Capitalize => {
             let original = string[first_character_position..].to_owned();
             string.truncate(first_character_position);
 
diff --git a/components/layout_thread/dom_wrapper.rs b/components/layout_thread/dom_wrapper.rs
index 88eedef7d17..301a7500713 100644
--- a/components/layout_thread/dom_wrapper.rs
+++ b/components/layout_thread/dom_wrapper.rs
@@ -973,6 +973,11 @@ impl<'le> ::selectors::Element for ServoLayoutElement<'le> {
     }
 
     #[inline]
+    fn is_part(&self, _name: &Atom) -> bool {
+        false
+    }
+
+    #[inline]
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         unsafe { self.element.has_class_for_layout(name, case_sensitivity) }
     }
@@ -1484,6 +1489,12 @@ impl<'le> ::selectors::Element for ServoThreadSafeLayoutElement<'le> {
         false
     }
 
+    #[inline]
+    fn is_part(&self, _name: &Atom) -> bool {
+        debug!("ServoThreadSafeLayoutElement::is_part called");
+        false
+    }
+
     fn has_class(&self, _name: &Atom, _case_sensitivity: CaseSensitivity) -> bool {
         debug!("ServoThreadSafeLayoutElement::has_class called");
         false
diff --git a/components/malloc_size_of/lib.rs b/components/malloc_size_of/lib.rs
index b59e26391ef..d65f8e957e4 100644
--- a/components/malloc_size_of/lib.rs
+++ b/components/malloc_size_of/lib.rs
@@ -747,6 +747,7 @@ where
             Component::ExplicitUniversalType |
             Component::LocalName(..) |
             Component::ID(..) |
+            Component::Part(..) |
             Component::Class(..) |
             Component::AttributeInNoNamespaceExists { .. } |
             Component::AttributeInNoNamespace { .. } |
@@ -776,12 +777,6 @@ impl<Impl: selectors::parser::SelectorImpl> MallocSizeOf
     }
 }
 
-impl MallocSizeOf for selectors::context::QuirksMode {
-    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
-        0
-    }
-}
-
 impl MallocSizeOf for Void {
     #[inline]
     fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 2d12e93774d..1b918c0537b 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -3075,6 +3075,10 @@ impl<'a> SelectorsElement for DomRoot<Element> {
             .map_or(false, |atom| case_sensitivity.eq_atom(id, atom))
     }
 
+    fn is_part(&self, _name: &Atom) -> bool {
+        false
+    }
+
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         Element::has_class(&**self, name, case_sensitivity)
     }
diff --git a/components/selectors/builder.rs b/components/selectors/builder.rs
index 72404de1084..b548ceb83ce 100644
--- a/components/selectors/builder.rs
+++ b/components/selectors/builder.rs
@@ -270,7 +270,7 @@ where
             Component::Combinator(..) => {
                 unreachable!("Found combinator in simple selectors vector?");
             },
-            Component::PseudoElement(..) | Component::LocalName(..) => {
+            Component::Part(..) | Component::PseudoElement(..) | Component::LocalName(..) => {
                 specificity.element_selectors += 1
             },
             Component::Slotted(ref selector) => {
diff --git a/components/selectors/context.rs b/components/selectors/context.rs
index 3ecd4dd82d5..95caa4055d7 100644
--- a/components/selectors/context.rs
+++ b/components/selectors/context.rs
@@ -279,13 +279,13 @@ where
     /// Runs F with a given shadow host which is the root of the tree whose
     /// rules we're matching.
     #[inline]
-    pub fn with_shadow_host<F, E, R>(&mut self, host: Option<E>, f: F) -> R
+    pub fn with_shadow_host<F, E, R>(&mut self, host: E, f: F) -> R
     where
         E: Element,
         F: FnOnce(&mut Self) -> R,
     {
         let original_host = self.current_host.take();
-        self.current_host = host.map(|h| h.opaque());
+        self.current_host = Some(host.opaque());
         let result = f(self);
         self.current_host = original_host;
         result
diff --git a/components/selectors/matching.rs b/components/selectors/matching.rs
index f50dc8ddecf..aa200cc4b26 100644
--- a/components/selectors/matching.rs
+++ b/components/selectors/matching.rs
@@ -450,6 +450,7 @@ where
 
             element.containing_shadow_host()
         },
+        Combinator::Part => element.containing_shadow_host(),
         Combinator::SlotAssignment => {
             debug_assert!(
                 context.current_host.is_some(),
@@ -517,6 +518,7 @@ where
         Combinator::Child |
         Combinator::Descendant |
         Combinator::SlotAssignment |
+        Combinator::Part |
         Combinator::PseudoElement => SelectorMatchingResult::NotMatchedGlobally,
     };
 
@@ -671,6 +673,7 @@ where
 
     match *selector {
         Component::Combinator(_) => unreachable!(),
+        Component::Part(ref part) => element.is_part(part),
         Component::Slotted(ref selector) => {
             // <slots> are never flattened tree slottables.
             !element.is_html_slot_element() &&
diff --git a/components/selectors/parser.rs b/components/selectors/parser.rs
index 0515eaca193..a924bbc17d0 100644
--- a/components/selectors/parser.rs
+++ b/components/selectors/parser.rs
@@ -30,10 +30,7 @@ pub trait PseudoElement: Sized + ToCss {
 
     /// Whether the pseudo-element supports a given state selector to the right
     /// of it.
-    fn supports_pseudo_class(
-        &self,
-        _pseudo_class: &<Self::Impl as SelectorImpl>::NonTSPseudoClass,
-    ) -> bool {
+    fn accepts_state_pseudo_classes(&self) -> bool {
         false
     }
 
@@ -50,6 +47,11 @@ pub trait NonTSPseudoClass: Sized + ToCss {
 
     /// Whether this pseudo-class is :active or :hover.
     fn is_active_or_hover(&self) -> bool;
+
+    /// Whether this pseudo-class belongs to:
+    ///
+    /// https://drafts.csswg.org/selectors-4/#useraction-pseudos
+    fn is_user_action_state(&self) -> bool;
 }
 
 /// Returns a Cow::Borrowed if `s` is already ASCII lowercase, and a
@@ -64,6 +66,72 @@ fn to_ascii_lowercase(s: &str) -> Cow<str> {
     }
 }
 
+bitflags! {
+    /// Flags that indicate at which point of parsing a selector are we.
+    struct SelectorParsingState: u8 {
+        /// Whether we're inside a negation. If we're inside a negation, we're
+        /// not allowed to add another negation or such, for example.
+        const INSIDE_NEGATION = 1 << 0;
+        /// Whether we've parsed a ::slotted() pseudo-element already.
+        ///
+        /// If so, then we can only parse a subset of pseudo-elements, and
+        /// whatever comes after them if so.
+        const AFTER_SLOTTED = 1 << 1;
+        /// Whether we've parsed a ::part() pseudo-element already.
+        ///
+        /// If so, then we can only parse a subset of pseudo-elements, and
+        /// whatever comes after them if so.
+        const AFTER_PART = 1 << 2;
+        /// Whether we've parsed a pseudo-element (as in, an
+        /// `Impl::PseudoElement` thus not accounting for `::slotted` or
+        /// `::part`) already.
+        ///
+        /// If so, then other pseudo-elements and most other selectors are
+        /// disallowed.
+        const AFTER_PSEUDO_ELEMENT = 1 << 3;
+        /// Whether we've parsed a non-stateful pseudo-element (again, as-in
+        /// `Impl::PseudoElement`) already. If so, then other pseudo-classes are
+        /// disallowed. If this flag is set, `AFTER_PSEUDO_ELEMENT` must be set
+        /// as well.
+        const AFTER_NON_STATEFUL_PSEUDO_ELEMENT = 1 << 4;
+        /// Whether we are after any of the pseudo-like things.
+        const AFTER_PSEUDO = Self::AFTER_PART.bits | Self::AFTER_SLOTTED.bits | Self::AFTER_PSEUDO_ELEMENT.bits;
+    }
+}
+
+impl SelectorParsingState {
+    #[inline]
+    fn allows_functional_pseudo_classes(self) -> bool {
+        !self.intersects(SelectorParsingState::AFTER_PSEUDO)
+    }
+
+    #[inline]
+    fn allows_slotted(self) -> bool {
+        !self.intersects(SelectorParsingState::AFTER_PSEUDO)
+    }
+
+    // TODO(emilio): Should we allow other ::part()s after ::part()?
+    //
+    // See https://github.com/w3c/csswg-drafts/issues/3841
+    #[inline]
+    fn allows_part(self) -> bool {
+        !self.intersects(SelectorParsingState::AFTER_PSEUDO)
+    }
+
+    #[inline]
+    fn allows_non_functional_pseudo_classes(self) -> bool {
+        !self.intersects(
+            SelectorParsingState::AFTER_SLOTTED |
+                SelectorParsingState::AFTER_NON_STATEFUL_PSEUDO_ELEMENT,
+        )
+    }
+
+    #[inline]
+    fn allows_tree_structural_pseudo_classes(self) -> bool {
+        !self.intersects(SelectorParsingState::AFTER_PSEUDO)
+    }
+}
+
 pub type SelectorParseError<'i> = ParseError<'i, SelectorParseErrorKind<'i>>;
 
 #[derive(Clone, Debug, PartialEq)]
@@ -76,6 +144,7 @@ pub enum SelectorParseErrorKind<'i> {
     NonCompoundSelector,
     NonPseudoElementAfterSlotted,
     InvalidPseudoElementAfterSlotted,
+    InvalidState,
     UnexpectedTokenInAttributeSelector(Token<'i>),
     PseudoElementExpectedColon(Token<'i>),
     PseudoElementExpectedIdent(Token<'i>),
@@ -108,6 +177,7 @@ macro_rules! with_all_bounds {
             type AttrValue: $($InSelector)*;
             type Identifier: $($InSelector)*;
             type ClassName: $($InSelector)*;
+            type PartName: $($InSelector)*;
             type LocalName: $($InSelector)* + Borrow<Self::BorrowedLocalName>;
             type NamespaceUrl: $($CommonBounds)* + Default + Borrow<Self::BorrowedNamespaceUrl>;
             type NamespacePrefix: $($InSelector)* + Default;
@@ -143,17 +213,16 @@ pub trait Parser<'i> {
     type Impl: SelectorImpl;
     type Error: 'i + From<SelectorParseErrorKind<'i>>;
 
-    /// Whether the name is a pseudo-element that can be specified with
-    /// the single colon syntax in addition to the double-colon syntax.
-    fn pseudo_element_allows_single_colon(name: &str) -> bool {
-        is_css2_pseudo_element(name)
-    }
-
     /// Whether to parse the `::slotted()` pseudo-element.
     fn parse_slotted(&self) -> bool {
         false
     }
 
+    /// Whether to parse the `::part()` pseudo-element.
+    fn parse_part(&self) -> bool {
+        false
+    }
+
     /// Whether to parse the `:host` pseudo-class.
     fn parse_host(&self) -> bool {
         false
@@ -694,7 +763,8 @@ impl<'a, Impl: 'a + SelectorImpl> SelectorIter<'a, Impl> {
     /// combinators to the left.
     #[inline]
     pub(crate) fn is_featureless_host_selector(&mut self) -> bool {
-        self.all(|component| matches!(*component, Component::Host(..))) &&
+        self.selector_length() > 0 &&
+            self.all(|component| matches!(*component, Component::Host(..))) &&
             self.next_sequence().is_none()
     }
 
@@ -798,6 +868,9 @@ pub enum Combinator {
     /// Another combinator used for ::slotted(), which represent the jump from
     /// a node to its assigned slot.
     SlotAssignment,
+    /// Another combinator used for `::part()`, which represents the jump from
+    /// the part to the containing shadow host.
+    Part,
 }
 
 impl Combinator {
@@ -891,8 +964,7 @@ pub enum Component<Impl: SelectorImpl> {
     LastOfType,
     OnlyOfType,
     NonTSPseudoClass(#[shmem(field_bound)] Impl::NonTSPseudoClass),
-    /// The ::slotted() pseudo-element (which isn't actually a pseudo-element,
-    /// and probably should be a pseudo-class):
+    /// The ::slotted() pseudo-element:
     ///
     /// https://drafts.csswg.org/css-scoping/#slotted-pseudo
     ///
@@ -904,6 +976,9 @@ pub enum Component<Impl: SelectorImpl> {
     ///
     /// See https://github.com/w3c/csswg-drafts/issues/2158
     Slotted(Selector<Impl>),
+    /// The `::part` pseudo-element.
+    ///   https://drafts.csswg.org/css-shadow-parts/#part
+    Part(#[shmem(field_bound)] Impl::PartName),
     /// The `:host` pseudo-class:
     ///
     /// https://drafts.csswg.org/css-scoping/#host-selector
@@ -1153,8 +1228,7 @@ impl ToCss for Combinator {
             Combinator::Descendant => dest.write_str(" "),
             Combinator::NextSibling => dest.write_str(" + "),
             Combinator::LaterSibling => dest.write_str(" ~ "),
-            Combinator::PseudoElement => Ok(()),
-            Combinator::SlotAssignment => Ok(()),
+            Combinator::PseudoElement | Combinator::Part | Combinator::SlotAssignment => Ok(()),
         }
     }
 }
@@ -1193,6 +1267,11 @@ impl<Impl: SelectorImpl> ToCss for Component<Impl> {
                 selector.to_css(dest)?;
                 dest.write_char(')')
             },
+            Part(ref part_name) => {
+                dest.write_str("::part(")?;
+                display_to_css_identifier(part_name, dest)?;
+                dest.write_char(')')
+            },
             PseudoElement(ref p) => p.to_css(dest),
             ID(ref s) => {
                 dest.write_char('#')?;
@@ -1364,15 +1443,12 @@ where
 {
     let mut builder = SelectorBuilder::default();
 
-    let mut has_pseudo_element;
-    let mut slotted;
+    let mut has_pseudo_element = false;
+    let mut slotted = false;
     'outer_loop: loop {
         // Parse a sequence of simple selectors.
-        match parse_compound_selector(parser, input, &mut builder)? {
-            Some((has_pseudo, slot)) => {
-                has_pseudo_element = has_pseudo;
-                slotted = slot;
-            },
+        let state = match parse_compound_selector(parser, input, &mut builder)? {
+            Some(state) => state,
             None => {
                 return Err(input.new_custom_error(if builder.has_combinators() {
                     SelectorParseErrorKind::DanglingCombinator
@@ -1382,7 +1458,11 @@ where
             },
         };
 
-        if has_pseudo_element || slotted {
+        if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+            has_pseudo_element = state.intersects(SelectorParsingState::AFTER_PSEUDO_ELEMENT);
+            slotted = state.intersects(SelectorParsingState::AFTER_SLOTTED);
+            let part = state.intersects(SelectorParsingState::AFTER_PART);
+            debug_assert!(has_pseudo_element || slotted || part);
             break;
         }
 
@@ -1420,6 +1500,8 @@ where
         builder.push_combinator(combinator);
     }
 
+    // TODO(emilio): We'll have to flag part() somehow as well, but we need more
+    // bits!
     Ok(Selector(builder.build(has_pseudo_element, slotted)))
 }
 
@@ -1510,6 +1592,7 @@ enum SimpleSelectorParseResult<Impl: SelectorImpl> {
     SimpleSelector(Component<Impl>),
     PseudoElement(Impl::PseudoElement),
     SlottedPseudo(Selector<Impl>),
+    PartPseudo(Impl::PartName),
 }
 
 #[derive(Debug)]
@@ -1848,7 +1931,7 @@ where
         Err(e) => return Err(e.into()),
     };
     if !is_type_sel {
-        match parse_one_simple_selector(parser, input, /* inside_negation = */ true)? {
+        match parse_one_simple_selector(parser, input, SelectorParsingState::INSIDE_NEGATION)? {
             Some(SimpleSelectorParseResult::SimpleSelector(s)) => {
                 sequence.push(s);
             },
@@ -1856,6 +1939,7 @@ where
                 return Err(input.new_custom_error(SelectorParseErrorKind::EmptyNegation));
             },
             Some(SimpleSelectorParseResult::PseudoElement(_)) |
+            Some(SimpleSelectorParseResult::PartPseudo(_)) |
             Some(SimpleSelectorParseResult::SlottedPseudo(_)) => {
                 let e = SelectorParseErrorKind::NonSimpleSelectorInNegation;
                 return Err(input.new_custom_error(e));
@@ -1875,14 +1959,11 @@ where
 ///
 /// `Err(())` means invalid selector.
 /// `Ok(None)` is an empty selector
-///
-/// The booleans represent whether a pseudo-element has been parsed, and whether
-/// ::slotted() has been parsed, respectively.
 fn parse_compound_selector<'i, 't, P, Impl>(
     parser: &P,
     input: &mut CssParser<'i, 't>,
     builder: &mut SelectorBuilder<Impl>,
-) -> Result<Option<(bool, bool)>, ParseError<'i, P::Error>>
+) -> Result<Option<SelectorParsingState>, ParseError<'i, P::Error>>
 where
     P: Parser<'i, Impl = Impl>,
     Impl: SelectorImpl,
@@ -1901,122 +1982,48 @@ where
         empty = false;
     }
 
-    let mut pseudo = false;
-    let mut slot = false;
+    let mut state = SelectorParsingState::empty();
     loop {
-        let parse_result =
-            match parse_one_simple_selector(parser, input, /* inside_negation = */ false)? {
-                None => break,
-                Some(result) => result,
-            };
+        let parse_result = match parse_one_simple_selector(parser, input, state)? {
+            None => break,
+            Some(result) => result,
+        };
 
         empty = false;
 
-        let slotted_selector;
-        let pseudo_element;
-
         match parse_result {
             SimpleSelectorParseResult::SimpleSelector(s) => {
                 builder.push_simple_selector(s);
-                continue;
             },
-            SimpleSelectorParseResult::PseudoElement(p) => {
-                slotted_selector = None;
-                pseudo_element = Some(p);
+            SimpleSelectorParseResult::PartPseudo(part_name) => {
+                state.insert(SelectorParsingState::AFTER_PART);
+                builder.push_combinator(Combinator::Part);
+                builder.push_simple_selector(Component::Part(part_name));
             },
             SimpleSelectorParseResult::SlottedPseudo(selector) => {
-                slotted_selector = Some(selector);
-                let maybe_pseudo =
-                    parse_one_simple_selector(parser, input, /* inside_negation = */ false)?;
-
-                pseudo_element = match maybe_pseudo {
-                    None => None,
-                    Some(SimpleSelectorParseResult::PseudoElement(pseudo)) => {
-                        if !pseudo.valid_after_slotted() {
-                            return Err(input.new_custom_error(
-                                SelectorParseErrorKind::InvalidPseudoElementAfterSlotted,
-                            ));
-                        }
-                        Some(pseudo)
-                    },
-                    Some(SimpleSelectorParseResult::SimpleSelector(..)) |
-                    Some(SimpleSelectorParseResult::SlottedPseudo(..)) => {
-                        return Err(input.new_custom_error(
-                            SelectorParseErrorKind::NonPseudoElementAfterSlotted,
-                        ));
-                    },
-                };
+                state.insert(SelectorParsingState::AFTER_SLOTTED);
+                if !builder.is_empty() {
+                    builder.push_combinator(Combinator::SlotAssignment);
+                }
+                builder.push_simple_selector(Component::Slotted(selector));
             },
-        }
-
-        debug_assert!(slotted_selector.is_some() || pseudo_element.is_some());
-        // Try to parse state to the right of the pseudo-element.
-        //
-        // There are only 3 allowable state selectors that can go on
-        // pseudo-elements as of right now.
-        let mut state_selectors = SmallVec::<[Component<Impl>; 3]>::new();
-        if let Some(ref p) = pseudo_element {
-            loop {
-                let location = input.current_source_location();
-                match input.next_including_whitespace() {
-                    Ok(&Token::Colon) => {},
-                    Ok(&Token::WhiteSpace(_)) | Err(_) => break,
-                    Ok(t) => {
-                        let e = SelectorParseErrorKind::PseudoElementExpectedColon(t.clone());
-                        return Err(location.new_custom_error(e));
-                    },
+            SimpleSelectorParseResult::PseudoElement(p) => {
+                state.insert(SelectorParsingState::AFTER_PSEUDO_ELEMENT);
+                if !p.accepts_state_pseudo_classes() {
+                    state.insert(SelectorParsingState::AFTER_NON_STATEFUL_PSEUDO_ELEMENT);
                 }
-
-                let location = input.current_source_location();
-                // TODO(emilio): Functional pseudo-classes too?
-                // We don't need it for now.
-                let name = match input.next_including_whitespace()? {
-                    &Token::Ident(ref name) => name.clone(),
-                    t => {
-                        return Err(location.new_custom_error(
-                            SelectorParseErrorKind::NoIdentForPseudo(t.clone()),
-                        ));
-                    },
-                };
-
-                let pseudo_class = P::parse_non_ts_pseudo_class(parser, location, name.clone())?;
-                if !p.supports_pseudo_class(&pseudo_class) {
-                    return Err(input.new_custom_error(
-                        SelectorParseErrorKind::UnsupportedPseudoClassOrElement(name),
-                    ));
+                if !builder.is_empty() {
+                    builder.push_combinator(Combinator::PseudoElement);
                 }
-                state_selectors.push(Component::NonTSPseudoClass(pseudo_class));
-            }
-        }
-
-        if let Some(slotted) = slotted_selector {
-            slot = true;
-            if !builder.is_empty() {
-                builder.push_combinator(Combinator::SlotAssignment);
-            }
-            builder.push_simple_selector(Component::Slotted(slotted));
-        }
-
-        if let Some(p) = pseudo_element {
-            pseudo = true;
-            if !builder.is_empty() {
-                builder.push_combinator(Combinator::PseudoElement);
-            }
-
-            builder.push_simple_selector(Component::PseudoElement(p));
-
-            for state_selector in state_selectors.drain() {
-                builder.push_simple_selector(state_selector);
-            }
+                builder.push_simple_selector(Component::PseudoElement(p));
+            },
         }
-
-        break;
     }
     if empty {
         // An empty selector is invalid.
         Ok(None)
     } else {
-        Ok(Some((pseudo, slot)))
+        Ok(Some(state))
     }
 }
 
@@ -2024,12 +2031,16 @@ fn parse_functional_pseudo_class<'i, 't, P, Impl>(
     parser: &P,
     input: &mut CssParser<'i, 't>,
     name: CowRcStr<'i>,
-    inside_negation: bool,
+    state: SelectorParsingState,
 ) -> Result<Component<Impl>, ParseError<'i, P::Error>>
 where
     P: Parser<'i, Impl = Impl>,
     Impl: SelectorImpl,
 {
+    if !state.allows_functional_pseudo_classes() {
+        return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+    debug_assert!(state.allows_tree_structural_pseudo_classes());
     match_ignore_ascii_case! { &name,
         "nth-child" => return Ok(parse_nth_pseudo_class(input, Component::NthChild)?),
         "nth-of-type" => return Ok(parse_nth_pseudo_class(input, Component::NthOfType)?),
@@ -2037,11 +2048,12 @@ where
         "nth-last-of-type" => return Ok(parse_nth_pseudo_class(input, Component::NthLastOfType)?),
         "host" => return Ok(Component::Host(Some(parse_inner_compound_selector(parser, input)?))),
         "not" => {
-            if inside_negation {
+            if state.intersects(SelectorParsingState::INSIDE_NEGATION) {
                 return Err(input.new_custom_error(
                     SelectorParseErrorKind::UnexpectedIdent("not".into())
                 ));
             }
+            debug_assert!(state.is_empty());
             return parse_negation(parser, input)
         },
         _ => {}
@@ -2064,7 +2076,7 @@ where
 /// Returns whether the name corresponds to a CSS2 pseudo-element that
 /// can be specified with the single colon syntax (in addition to the
 /// double-colon syntax, which can be used for all pseudo-elements).
-pub fn is_css2_pseudo_element(name: &str) -> bool {
+fn is_css2_pseudo_element(name: &str) -> bool {
     // ** Do not add to this list! **
     match_ignore_ascii_case! { name,
         "before" | "after" | "first-line" | "first-letter" => true,
@@ -2080,37 +2092,52 @@ pub fn is_css2_pseudo_element(name: &str) -> bool {
 fn parse_one_simple_selector<'i, 't, P, Impl>(
     parser: &P,
     input: &mut CssParser<'i, 't>,
-    inside_negation: bool,
+    state: SelectorParsingState,
 ) -> Result<Option<SimpleSelectorParseResult<Impl>>, ParseError<'i, P::Error>>
 where
     P: Parser<'i, Impl = Impl>,
     Impl: SelectorImpl,
 {
     let start = input.state();
-    // FIXME: remove clone() when lifetimes are non-lexical
-    match input.next_including_whitespace().map(|t| t.clone()) {
-        Ok(Token::IDHash(id)) => {
+    let token = match input.next_including_whitespace().map(|t| t.clone()) {
+        Ok(t) => t,
+        Err(..) => {
+            input.reset(&start);
+            return Ok(None);
+        },
+    };
+
+    Ok(Some(match token {
+        Token::IDHash(id) => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
             let id = Component::ID(id.as_ref().into());
-            Ok(Some(SimpleSelectorParseResult::SimpleSelector(id)))
+            SimpleSelectorParseResult::SimpleSelector(id)
         },
-        Ok(Token::Delim('.')) => {
+        Token::Delim('.') => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
             let location = input.current_source_location();
-            match *input.next_including_whitespace()? {
-                Token::Ident(ref class) => {
-                    let class = Component::Class(class.as_ref().into());
-                    Ok(Some(SimpleSelectorParseResult::SimpleSelector(class)))
-                },
+            let class = match *input.next_including_whitespace()? {
+                Token::Ident(ref class) => class,
                 ref t => {
                     let e = SelectorParseErrorKind::ClassNeedsIdent(t.clone());
-                    Err(location.new_custom_error(e))
+                    return Err(location.new_custom_error(e));
                 },
-            }
+            };
+            let class = Component::Class(class.as_ref().into());
+            SimpleSelectorParseResult::SimpleSelector(class)
         },
-        Ok(Token::SquareBracketBlock) => {
+        Token::SquareBracketBlock => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
             let attr = input.parse_nested_block(|input| parse_attribute_selector(parser, input))?;
-            Ok(Some(SimpleSelectorParseResult::SimpleSelector(attr)))
+            SimpleSelectorParseResult::SimpleSelector(attr)
         },
-        Ok(Token::Colon) => {
+        Token::Colon => {
             let location = input.current_source_location();
             let (is_single_colon, next_token) = match input.next_including_whitespace()?.clone() {
                 Token::Colon => (false, input.next_including_whitespace()?.clone()),
@@ -2124,72 +2151,102 @@ where
                     return Err(input.new_custom_error(e));
                 },
             };
-            let is_pseudo_element =
-                !is_single_colon || P::pseudo_element_allows_single_colon(&name);
+            let is_pseudo_element = !is_single_colon || is_css2_pseudo_element(&name);
             if is_pseudo_element {
-                let parse_result = if is_functional {
+                if state.intersects(SelectorParsingState::AFTER_PSEUDO_ELEMENT) {
+                    return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+                }
+                let pseudo_element = if is_functional {
+                    if P::parse_part(parser) && name.eq_ignore_ascii_case("part") {
+                        if !state.allows_part() {
+                            return Err(
+                                input.new_custom_error(SelectorParseErrorKind::InvalidState)
+                            );
+                        }
+                        let name = input.parse_nested_block(|input| {
+                            Ok(input.expect_ident()?.as_ref().into())
+                        })?;
+                        return Ok(Some(SimpleSelectorParseResult::PartPseudo(name)));
+                    }
                     if P::parse_slotted(parser) && name.eq_ignore_ascii_case("slotted") {
+                        if !state.allows_slotted() {
+                            return Err(
+                                input.new_custom_error(SelectorParseErrorKind::InvalidState)
+                            );
+                        }
                         let selector = input.parse_nested_block(|input| {
                             parse_inner_compound_selector(parser, input)
                         })?;
-                        SimpleSelectorParseResult::SlottedPseudo(selector)
-                    } else {
-                        let selector = input.parse_nested_block(|input| {
-                            P::parse_functional_pseudo_element(parser, name, input)
-                        })?;
-                        SimpleSelectorParseResult::PseudoElement(selector)
+                        return Ok(Some(SimpleSelectorParseResult::SlottedPseudo(selector)));
                     }
+                    input.parse_nested_block(|input| {
+                        P::parse_functional_pseudo_element(parser, name, input)
+                    })?
                 } else {
-                    SimpleSelectorParseResult::PseudoElement(P::parse_pseudo_element(
-                        parser, location, name,
-                    )?)
+                    P::parse_pseudo_element(parser, location, name)?
                 };
-                Ok(Some(parse_result))
+
+                if state.intersects(SelectorParsingState::AFTER_SLOTTED) &&
+                    !pseudo_element.valid_after_slotted()
+                {
+                    return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+                }
+                SimpleSelectorParseResult::PseudoElement(pseudo_element)
             } else {
                 let pseudo_class = if is_functional {
                     input.parse_nested_block(|input| {
-                        parse_functional_pseudo_class(parser, input, name, inside_negation)
+                        parse_functional_pseudo_class(parser, input, name, state)
                     })?
                 } else {
-                    parse_simple_pseudo_class(parser, location, name)?
+                    parse_simple_pseudo_class(parser, location, name, state)?
                 };
-                Ok(Some(SimpleSelectorParseResult::SimpleSelector(
-                    pseudo_class,
-                )))
+                SimpleSelectorParseResult::SimpleSelector(pseudo_class)
             }
         },
         _ => {
             input.reset(&start);
-            Ok(None)
+            return Ok(None);
         },
-    }
+    }))
 }
 
 fn parse_simple_pseudo_class<'i, P, Impl>(
     parser: &P,
     location: SourceLocation,
     name: CowRcStr<'i>,
+    state: SelectorParsingState,
 ) -> Result<Component<Impl>, ParseError<'i, P::Error>>
 where
     P: Parser<'i, Impl = Impl>,
     Impl: SelectorImpl,
 {
-    (match_ignore_ascii_case! { &name,
-        "first-child" => Ok(Component::FirstChild),
-        "last-child"  => Ok(Component::LastChild),
-        "only-child"  => Ok(Component::OnlyChild),
-        "root" => Ok(Component::Root),
-        "empty" => Ok(Component::Empty),
-        "scope" => Ok(Component::Scope),
-        "host" if P::parse_host(parser) => Ok(Component::Host(None)),
-        "first-of-type" => Ok(Component::FirstOfType),
-        "last-of-type" => Ok(Component::LastOfType),
-        "only-of-type" => Ok(Component::OnlyOfType),
-        _ => Err(())
-    })
-    .or_else(|()| {
-        P::parse_non_ts_pseudo_class(parser, location, name).map(Component::NonTSPseudoClass)
-    })
+    if !state.allows_non_functional_pseudo_classes() {
+        return Err(location.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+
+    if state.allows_tree_structural_pseudo_classes() {
+        match_ignore_ascii_case! { &name,
+            "first-child" => return Ok(Component::FirstChild),
+            "last-child" => return Ok(Component::LastChild),
+            "only-child" => return Ok(Component::OnlyChild),
+            "root" => return Ok(Component::Root),
+            "empty" => return Ok(Component::Empty),
+            "scope" => return Ok(Component::Scope),
+            "host" if P::parse_host(parser) => return Ok(Component::Host(None)),
+            "first-of-type" => return Ok(Component::FirstOfType),
+            "last-of-type" => return Ok(Component::LastOfType),
+            "only-of-type" => return Ok(Component::OnlyOfType),
+            _ => {},
+        }
+    }
+
+    let pseudo_class = P::parse_non_ts_pseudo_class(parser, location, name)?;
+    if state.intersects(SelectorParsingState::AFTER_PSEUDO_ELEMENT) &&
+        !pseudo_class.is_user_action_state()
+    {
+        return Err(location.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+    Ok(Component::NonTSPseudoClass(pseudo_class))
 }
 
 // NB: pub module in order to access the DummyParser
@@ -2218,11 +2275,8 @@ pub mod tests {
     impl parser::PseudoElement for PseudoElement {
         type Impl = DummySelectorImpl;
 
-        fn supports_pseudo_class(&self, pc: &PseudoClass) -> bool {
-            match *pc {
-                PseudoClass::Hover => true,
-                PseudoClass::Active | PseudoClass::Lang(..) => false,
-            }
+        fn accepts_state_pseudo_classes(&self) -> bool {
+            true
         }
 
         fn valid_after_slotted(&self) -> bool {
@@ -2237,6 +2291,11 @@ pub mod tests {
         fn is_active_or_hover(&self) -> bool {
             matches!(*self, PseudoClass::Active | PseudoClass::Hover)
         }
+
+        #[inline]
+        fn is_user_action_state(&self) -> bool {
+            self.is_active_or_hover()
+        }
     }
 
     impl ToCss for PseudoClass {
@@ -2302,6 +2361,7 @@ pub mod tests {
         type AttrValue = DummyAtom;
         type Identifier = DummyAtom;
         type ClassName = DummyAtom;
+        type PartName = DummyAtom;
         type LocalName = DummyAtom;
         type NamespaceUrl = DummyAtom;
         type NamespacePrefix = DummyAtom;
@@ -2340,6 +2400,10 @@ pub mod tests {
             true
         }
 
+        fn parse_part(&self) -> bool {
+            true
+        }
+
         fn parse_non_ts_pseudo_class(
             &self,
             location: SourceLocation,
@@ -2789,11 +2853,11 @@ pub mod tests {
                 specificity(0, 2, 1) | HAS_PSEUDO_BIT,
             )]))
         );
-        assert!(parse("::before:hover:active").is_err());
+        assert!(parse("::before:hover:lang(foo)").is_err());
         assert!(parse("::before:hover .foo").is_err());
         assert!(parse("::before .foo").is_err());
         assert!(parse("::before ~ bar").is_err());
-        assert!(parse("::before:active").is_err());
+        assert!(parse("::before:active").is_ok());
 
         // https://github.com/servo/servo/issues/15335
         assert!(parse(":: before").is_err());
@@ -2914,6 +2978,14 @@ pub mod tests {
         assert!(parse("::slotted(div).foo").is_err());
         assert!(parse("::slotted(div + bar)").is_err());
         assert!(parse("::slotted(div) + foo").is_err());
+
+        assert!(parse("::part()").is_err());
+        assert!(parse("::part(42)").is_err());
+        // Though note https://github.com/w3c/csswg-drafts/issues/3502
+        assert!(parse("::part(foo bar)").is_err());
+        assert!(parse("::part(foo):hover").is_ok());
+        assert!(parse("::part(foo) + bar").is_err());
+
         assert!(parse("div ::slotted(div)").is_ok());
         assert!(parse("div + slot::slotted(div)").is_ok());
         assert!(parse("div + slot::slotted(div.foo)").is_ok());
diff --git a/components/selectors/tree.rs b/components/selectors/tree.rs
index badfca86ed6..52599893d2f 100644
--- a/components/selectors/tree.rs
+++ b/components/selectors/tree.rs
@@ -110,6 +110,8 @@ pub trait Element: Sized + Clone + Debug {
         case_sensitivity: CaseSensitivity,
     ) -> bool;
 
+    fn is_part(&self, name: &<Self::Impl as SelectorImpl>::PartName) -> bool;
+
     /// Returns whether this element matches `:empty`.
     ///
     /// That is, whether it does not contain any child element or any non-zero-length text node.
diff --git a/components/style/author_styles.rs b/components/style/author_styles.rs
index eff64ed6c1e..58d9bda423a 100644
--- a/components/style/author_styles.rs
+++ b/components/style/author_styles.rs
@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 //! A set of author stylesheets and their computed representation, such as the
-//! ones used for ShadowRoot and XBL.
+//! ones used for ShadowRoot.
 
 use crate::context::QuirksMode;
 use crate::dom::TElement;
@@ -17,7 +17,7 @@ use crate::stylesheets::StylesheetInDocument;
 use crate::stylist::CascadeData;
 
 /// A set of author stylesheets and their computed representation, such as the
-/// ones used for ShadowRoot and XBL.
+/// ones used for ShadowRoot.
 #[derive(MallocSizeOf)]
 pub struct AuthorStyles<S>
 where
@@ -28,9 +28,6 @@ where
     pub stylesheets: AuthorStylesheetSet<S>,
     /// The actual cascade data computed from the stylesheets.
     pub data: CascadeData,
-    /// The quirks mode of the last stylesheet flush, used because XBL sucks and
-    /// we should really fix it, see bug 1406875.
-    pub quirks_mode: QuirksMode,
 }
 
 impl<S> AuthorStyles<S>
@@ -43,7 +40,6 @@ where
         Self {
             stylesheets: AuthorStylesheetSet::new(),
             data: CascadeData::new(),
-            quirks_mode: QuirksMode::NoQuirks,
         }
     }
 
@@ -65,10 +61,6 @@ where
             .stylesheets
             .flush::<E>(/* host = */ None, /* snapshot_map = */ None);
 
-        if flusher.sheets.dirty() {
-            self.quirks_mode = quirks_mode;
-        }
-
         // Ignore OOM.
         let _ = self
             .data
diff --git a/components/style/dom.rs b/components/style/dom.rs
index 967b978bad4..9a81e2ec774 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -761,7 +761,7 @@ pub trait TElement:
     /// Returns the anonymous content for the current element's XBL binding,
     /// given if any.
     ///
-    /// This is used in Gecko for XBL and shadow DOM.
+    /// This is used in Gecko for XBL.
     fn xbl_binding_anonymous_content(&self) -> Option<Self::ConcreteNode> {
         None
     }
@@ -772,11 +772,6 @@ pub trait TElement:
     /// The shadow root which roots the subtree this element is contained in.
     fn containing_shadow(&self) -> Option<<Self::ConcreteNode as TNode>::ConcreteShadowRoot>;
 
-    /// XBL hack for style sharing. :(
-    fn has_same_xbl_proto_binding_as(&self, _other: Self) -> bool {
-        true
-    }
-
     /// Return the element which we can use to look up rules in the selector
     /// maps.
     ///
@@ -792,56 +787,34 @@ pub trait TElement:
         }
     }
 
-    /// Implements Gecko's `nsBindingManager::WalkRules`.
-    ///
-    /// Returns whether to cut off the binding inheritance, that is, whether
-    /// document rules should _not_ apply.
-    fn each_xbl_cascade_data<'a, F>(&self, _: F) -> bool
-    where
-        Self: 'a,
-        F: FnMut(&'a CascadeData, QuirksMode),
-    {
-        false
-    }
-
     /// Executes the callback for each applicable style rule data which isn't
     /// the main document's data (which stores UA / author rules).
     ///
     /// The element passed to the callback is the containing shadow host for the
-    /// data if it comes from Shadow DOM, None if it comes from XBL.
+    /// data if it comes from Shadow DOM.
     ///
     /// Returns whether normal document author rules should apply.
     fn each_applicable_non_document_style_rule_data<'a, F>(&self, mut f: F) -> bool
     where
         Self: 'a,
-        F: FnMut(&'a CascadeData, QuirksMode, Option<Self>),
+        F: FnMut(&'a CascadeData, Self),
     {
         use rule_collector::containing_shadow_ignoring_svg_use;
 
-        let mut doc_rules_apply = !self.each_xbl_cascade_data(|data, quirks_mode| {
-            f(data, quirks_mode, None);
-        });
+        let mut doc_rules_apply = self.matches_user_and_author_rules();
 
         // Use the same rules to look for the containing host as we do for rule
         // collection.
         if let Some(shadow) = containing_shadow_ignoring_svg_use(*self) {
             doc_rules_apply = false;
             if let Some(data) = shadow.style_data() {
-                f(
-                    data,
-                    self.as_node().owner_doc().quirks_mode(),
-                    Some(shadow.host()),
-                );
+                f(data, shadow.host());
             }
         }
 
         if let Some(shadow) = self.shadow_root() {
             if let Some(data) = shadow.style_data() {
-                f(
-                    data,
-                    self.as_node().owner_doc().quirks_mode(),
-                    Some(shadow.host()),
-                );
+                f(data, shadow.host());
             }
         }
 
@@ -850,11 +823,7 @@ pub trait TElement:
             // Slots can only have assigned nodes when in a shadow tree.
             let shadow = slot.containing_shadow().unwrap();
             if let Some(data) = shadow.style_data() {
-                f(
-                    data,
-                    self.as_node().owner_doc().quirks_mode(),
-                    Some(shadow.host()),
-                );
+                f(data, shadow.host());
             }
             current = slot.assigned_slot();
         }
diff --git a/components/style/gecko/conversions.rs b/components/style/gecko/conversions.rs
index 43438438a40..4cd5722bdb0 100644
--- a/components/style/gecko/conversions.rs
+++ b/components/style/gecko/conversions.rs
@@ -13,18 +13,17 @@
 use crate::gecko::values::GeckoStyleCoordConvertible;
 use crate::gecko_bindings::bindings;
 use crate::gecko_bindings::structs::{self, nsStyleCoord_CalcValue, Matrix4x4Components};
-use crate::gecko_bindings::structs::{nsStyleImage, nsresult, SheetType};
+use crate::gecko_bindings::structs::{nsStyleImage, nsresult};
 use crate::gecko_bindings::sugar::ns_style_coord::{CoordData, CoordDataMut, CoordDataValue};
-use crate::stylesheets::{Origin, RulesMutateError};
+use crate::stylesheets::RulesMutateError;
 use crate::values::computed::image::LineDirection;
 use crate::values::computed::transform::Matrix3D;
 use crate::values::computed::url::ComputedImageUrl;
 use crate::values::computed::{Angle, Gradient, Image};
 use crate::values::computed::{Integer, LengthPercentage};
 use crate::values::computed::{Length, Percentage, TextAlign};
-use crate::values::generics::box_::VerticalAlign;
 use crate::values::generics::grid::{TrackListValue, TrackSize};
-use crate::values::generics::image::{CompatMode, GradientItem, Image as GenericImage};
+use crate::values::generics::image::{CompatMode, Image as GenericImage};
 use crate::values::generics::rect::Rect;
 use crate::Zero;
 use app_units::Au;
@@ -154,7 +153,6 @@ impl nsStyleImage {
 
     // FIXME(emilio): This is really complex, we should use cbindgen for this.
     fn set_gradient(&mut self, gradient: Gradient) {
-        use self::structs::nsStyleCoord;
         use self::structs::NS_STYLE_GRADIENT_SIZE_CLOSEST_CORNER as CLOSEST_CORNER;
         use self::structs::NS_STYLE_GRADIENT_SIZE_CLOSEST_SIDE as CLOSEST_SIDE;
         use self::structs::NS_STYLE_GRADIENT_SIZE_FARTHEST_CORNER as FARTHEST_CORNER;
@@ -329,26 +327,9 @@ impl nsStyleImage {
             },
         };
 
-        for (index, item) in gradient.items.iter().enumerate() {
-            // NB: stops are guaranteed to be none in the gecko side by
-            // default.
-
+        for (index, item) in gradient.items.into_iter().enumerate() {
             let gecko_stop = unsafe { &mut (*gecko_gradient).mStops[index] };
-            let mut coord = nsStyleCoord::null();
-
-            match *item {
-                GradientItem::ColorStop(ref stop) => {
-                    gecko_stop.mColor = stop.color.into();
-                    gecko_stop.mIsInterpolationHint = false;
-                    coord.set(stop.position);
-                },
-                GradientItem::InterpolationHint(hint) => {
-                    gecko_stop.mIsInterpolationHint = true;
-                    coord.set(Some(hint));
-                },
-            }
-
-            gecko_stop.mLocation.move_from(coord);
+            *gecko_stop = item;
         }
 
         unsafe {
@@ -419,7 +400,7 @@ impl nsStyleImage {
         use self::structs::NS_STYLE_GRADIENT_SIZE_FARTHEST_CORNER as FARTHEST_CORNER;
         use self::structs::NS_STYLE_GRADIENT_SIZE_FARTHEST_SIDE as FARTHEST_SIDE;
         use crate::values::computed::position::Position;
-        use crate::values::generics::image::{Circle, ColorStop, Ellipse};
+        use crate::values::generics::image::{Circle, Ellipse};
         use crate::values::generics::image::{EndingShape, GradientKind, ShapeExtent};
 
         let gecko_gradient = bindings::Gecko_GetGradientImageValue(self)
@@ -531,24 +512,7 @@ impl nsStyleImage {
             },
         };
 
-        let items = gecko_gradient
-            .mStops
-            .iter()
-            .map(|ref stop| {
-                if stop.mIsInterpolationHint {
-                    GradientItem::InterpolationHint(
-                        LengthPercentage::from_gecko_style_coord(&stop.mLocation)
-                            .expect("mLocation could not convert to LengthPercentage"),
-                    )
-                } else {
-                    GradientItem::ColorStop(ColorStop {
-                        color: stop.mColor.into(),
-                        position: LengthPercentage::from_gecko_style_coord(&stop.mLocation),
-                    })
-                }
-            })
-            .collect();
-
+        let items = gecko_gradient.mStops.iter().cloned().collect();
         let compat_mode = if gecko_gradient.mMozLegacySyntax {
             CompatMode::Moz
         } else if gecko_gradient.mLegacySyntax {
@@ -818,16 +782,6 @@ impl From<RulesMutateError> for nsresult {
     }
 }
 
-impl From<Origin> for SheetType {
-    fn from(other: Origin) -> Self {
-        match other {
-            Origin::UserAgent => SheetType::Agent,
-            Origin::Author => SheetType::Doc,
-            Origin::User => SheetType::User,
-        }
-    }
-}
-
 impl TrackSize<LengthPercentage> {
     /// Return TrackSize from given two nsStyleCoord
     pub fn from_gecko_style_coords<T: CoordData>(gecko_min: &T, gecko_max: &T) -> Self {
@@ -920,26 +874,6 @@ where
     }
 }
 
-impl<L> VerticalAlign<L> {
-    /// Converts an enumerated value coming from Gecko to a `VerticalAlign<L>`.
-    pub fn from_gecko_keyword(value: u32) -> Self {
-        match value {
-            structs::NS_STYLE_VERTICAL_ALIGN_BASELINE => VerticalAlign::Baseline,
-            structs::NS_STYLE_VERTICAL_ALIGN_SUB => VerticalAlign::Sub,
-            structs::NS_STYLE_VERTICAL_ALIGN_SUPER => VerticalAlign::Super,
-            structs::NS_STYLE_VERTICAL_ALIGN_TOP => VerticalAlign::Top,
-            structs::NS_STYLE_VERTICAL_ALIGN_TEXT_TOP => VerticalAlign::TextTop,
-            structs::NS_STYLE_VERTICAL_ALIGN_MIDDLE => VerticalAlign::Middle,
-            structs::NS_STYLE_VERTICAL_ALIGN_BOTTOM => VerticalAlign::Bottom,
-            structs::NS_STYLE_VERTICAL_ALIGN_TEXT_BOTTOM => VerticalAlign::TextBottom,
-            structs::NS_STYLE_VERTICAL_ALIGN_MIDDLE_WITH_BASELINE => {
-                VerticalAlign::MozMiddleWithBaseline
-            },
-            _ => panic!("unexpected enumerated value for vertical-align"),
-        }
-    }
-}
-
 impl TextAlign {
     /// Obtain a specified value from a Gecko keyword value
     ///
diff --git a/components/style/gecko/data.rs b/components/style/gecko/data.rs
index ead7d88502b..0ce43bca46d 100644
--- a/components/style/gecko/data.rs
+++ b/components/style/gecko/data.rs
@@ -145,11 +145,6 @@ impl PerDocumentStyleData {
     /// Create a `PerDocumentStyleData`.
     pub fn new(document: *const structs::Document) -> Self {
         let device = Device::new(document);
-
-        // FIXME(emilio, tlin): How is this supposed to work with XBL? This is
-        // right now not always honored, see bug 1405543...
-        //
-        // Should we just force XBL Stylists to be NoQuirks?
         let quirks_mode = device.document().mCompatMode;
 
         PerDocumentStyleData(AtomicRefCell::new(PerDocumentStyleDataImpl {
diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
index b39e748ce2b..4c04d9573e5 100644
--- a/components/style/gecko/media_queries.rs
+++ b/components/style/gecko/media_queries.rs
@@ -169,7 +169,6 @@ impl Device {
             self.document()
                 .mPresShell
                 .as_ref()?
-                ._base
                 .mPresContext
                 .mRawPtr
                 .as_ref()
diff --git a/components/style/gecko/pseudo_element.rs b/components/style/gecko/pseudo_element.rs
index 479c12b9cde..7538b785858 100644
--- a/components/style/gecko/pseudo_element.rs
+++ b/components/style/gecko/pseudo_element.rs
@@ -11,7 +11,7 @@
 use crate::gecko_bindings::structs::{self, PseudoStyleType};
 use crate::properties::longhands::display::computed_value::T as Display;
 use crate::properties::{ComputedValues, PropertyFlags};
-use crate::selector_parser::{NonTSPseudoClass, PseudoElementCascadeType, SelectorImpl};
+use crate::selector_parser::{PseudoElementCascadeType, SelectorImpl};
 use crate::str::{starts_with_ignore_ascii_case, string_as_ascii_lowercase};
 use crate::string_cache::Atom;
 use crate::values::serialize_atom_identifier;
@@ -30,6 +30,7 @@ impl ::selectors::parser::PseudoElement for PseudoElement {
     // ::slotted() should support all tree-abiding pseudo-elements, see
     // https://drafts.csswg.org/css-scoping/#slotted-pseudo
     // https://drafts.csswg.org/css-pseudo-4/#treelike
+    #[inline]
     fn valid_after_slotted(&self) -> bool {
         matches!(
             *self,
@@ -40,12 +41,9 @@ impl ::selectors::parser::PseudoElement for PseudoElement {
         )
     }
 
-    fn supports_pseudo_class(&self, pseudo_class: &NonTSPseudoClass) -> bool {
-        if !self.supports_user_action_state() {
-            return false;
-        }
-
-        return pseudo_class.is_safe_user_action_state();
+    #[inline]
+    fn accepts_state_pseudo_classes(&self) -> bool {
+        self.supports_user_action_state()
     }
 }
 
diff --git a/components/style/gecko/selector_parser.rs b/components/style/gecko/selector_parser.rs
index 41de05c33b9..18718446c1a 100644
--- a/components/style/gecko/selector_parser.rs
+++ b/components/style/gecko/selector_parser.rs
@@ -184,16 +184,6 @@ impl NonTSPseudoClass {
         }
     }
 
-    /// <https://drafts.csswg.org/selectors-4/#useraction-pseudos>
-    ///
-    /// We intentionally skip the link-related ones.
-    pub fn is_safe_user_action_state(&self) -> bool {
-        matches!(
-            *self,
-            NonTSPseudoClass::Hover | NonTSPseudoClass::Active | NonTSPseudoClass::Focus
-        )
-    }
-
     /// Get the state flag associated with a pseudo-class, if any.
     pub fn state_flag(&self) -> ElementState {
         macro_rules! flag {
@@ -279,6 +269,15 @@ impl ::selectors::parser::NonTSPseudoClass for NonTSPseudoClass {
     fn is_active_or_hover(&self) -> bool {
         matches!(*self, NonTSPseudoClass::Active | NonTSPseudoClass::Hover)
     }
+
+    /// We intentionally skip the link-related ones.
+    #[inline]
+    fn is_user_action_state(&self) -> bool {
+        matches!(
+            *self,
+            NonTSPseudoClass::Hover | NonTSPseudoClass::Active | NonTSPseudoClass::Focus
+        )
+    }
 }
 
 /// The dummy struct we use to implement our selector parsing.
@@ -290,6 +289,7 @@ impl ::selectors::SelectorImpl for SelectorImpl {
     type AttrValue = Atom;
     type Identifier = Atom;
     type ClassName = Atom;
+    type PartName = Atom;
     type LocalName = Atom;
     type NamespacePrefix = Atom;
     type NamespaceUrl = Namespace;
@@ -352,11 +352,6 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         self.parse_slotted()
     }
 
-    fn pseudo_element_allows_single_colon(name: &str) -> bool {
-        // FIXME: -moz-tree check should probably be ascii-case-insensitive.
-        ::selectors::parser::is_css2_pseudo_element(name) || name.starts_with("-moz-tree-")
-    }
-
     fn parse_non_ts_pseudo_class(
         &self,
         location: SourceLocation,
diff --git a/components/style/gecko/snapshot.rs b/components/style/gecko/snapshot.rs
index 6ac4003fa25..d294939baeb 100644
--- a/components/style/gecko/snapshot.rs
+++ b/components/style/gecko/snapshot.rs
@@ -184,12 +184,22 @@ impl ElementSnapshot for GeckoElementSnapshot {
     }
 
     #[inline]
+    fn is_part(&self, name: &Atom) -> bool {
+        let attr = match snapshot_helpers::find_attr(&*self.mAttrs, &atom!("part")) {
+            Some(attr) => attr,
+            None => return false,
+        };
+
+        snapshot_helpers::has_class_or_part(name, CaseSensitivity::CaseSensitive, attr)
+    }
+
+    #[inline]
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         if !self.has_any(Flags::MaybeClass) {
             return false;
         }
 
-        snapshot_helpers::has_class(name, case_sensitivity, &self.mClass)
+        snapshot_helpers::has_class_or_part(name, case_sensitivity, &self.mClass)
     }
 
     #[inline]
diff --git a/components/style/gecko/snapshot_helpers.rs b/components/style/gecko/snapshot_helpers.rs
index 0eb84830546..aaa2254dbd7 100644
--- a/components/style/gecko/snapshot_helpers.rs
+++ b/components/style/gecko/snapshot_helpers.rs
@@ -29,7 +29,7 @@ unsafe fn ptr<T>(attr: &structs::nsAttrValue) -> *const T {
 }
 
 #[inline(always)]
-unsafe fn get_class_from_attr(attr: &structs::nsAttrValue) -> Class {
+unsafe fn get_class_or_part_from_attr(attr: &structs::nsAttrValue) -> Class {
     debug_assert!(bindings::Gecko_AssertClassAttrValueIsSane(attr));
     let base_type = base_type(attr);
     if base_type == structs::nsAttrValue_ValueBaseType_eStringBase {
@@ -82,15 +82,15 @@ pub fn get_id(attrs: &[structs::AttrArray_InternalAttr]) -> Option<&WeakAtom> {
     Some(unsafe { get_id_from_attr(find_attr(attrs, &atom!("id"))?) })
 }
 
-/// Given a class name, a case sensitivity, and an array of attributes, returns
-/// whether the class has the attribute that name represents.
+/// Given a class or part name, a case sensitivity, and an array of attributes,
+/// returns whether the attribute has that name.
 #[inline(always)]
-pub fn has_class(
+pub fn has_class_or_part(
     name: &Atom,
     case_sensitivity: CaseSensitivity,
     attr: &structs::nsAttrValue,
 ) -> bool {
-    match unsafe { get_class_from_attr(attr) } {
+    match unsafe { get_class_or_part_from_attr(attr) } {
         Class::None => false,
         Class::One(atom) => unsafe { case_sensitivity.eq_atom(name, WeakAtom::new(atom)) },
         Class::More(atoms) => match case_sensitivity {
@@ -114,7 +114,7 @@ where
     F: FnMut(&Atom),
 {
     unsafe {
-        match get_class_from_attr(attr) {
+        match get_class_or_part_from_attr(attr) {
             Class::None => {},
             Class::One(atom) => Atom::with(atom, callback),
             Class::More(atoms) => {
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 0fa354fb139..9d987c56308 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -172,15 +172,7 @@ impl<'lr> TShadowRoot for GeckoShadowRoot<'lr> {
         Self: 'a,
     {
         let author_styles = unsafe { self.0.mServoStyles.mPtr.as_ref()? };
-
         let author_styles = AuthorStyles::<GeckoStyleSheet>::from_ffi(author_styles);
-
-        debug_assert!(
-            author_styles.quirks_mode == self.as_node().owner_doc().quirks_mode() ||
-                author_styles.stylesheets.is_empty() ||
-                author_styles.stylesheets.dirty()
-        );
-
         Some(&author_styles.data)
     }
 
@@ -536,11 +528,6 @@ impl<'lb> GeckoXBLBinding<'lb> {
         self.0.mContent.raw::<nsIContent>()
     }
 
-    #[inline]
-    fn inherits_style(&self) -> bool {
-        unsafe { bindings::Gecko_XBLBinding_InheritsStyle(self.0) }
-    }
-
     // This duplicates the logic in Gecko's
     // nsBindingManager::GetBindingWithContent.
     fn binding_with_content(&self) -> Option<Self> {
@@ -552,22 +539,6 @@ impl<'lb> GeckoXBLBinding<'lb> {
             binding = binding.base_binding()?;
         }
     }
-
-    fn each_xbl_cascade_data<F>(&self, f: &mut F)
-    where
-        F: FnMut(&'lb CascadeData, QuirksMode),
-    {
-        if let Some(base) = self.base_binding() {
-            base.each_xbl_cascade_data(f);
-        }
-
-        let data = unsafe { bindings::Gecko_XBLBinding_GetRawServoStyles(self.0).as_ref() };
-
-        if let Some(data) = data {
-            let data: &'lb _ = AuthorStyles::<GeckoStyleSheet>::from_ffi(data);
-            f(&data.data, data.quirks_mode)
-        }
-    }
 }
 
 /// A simple wrapper over a non-null Gecko `Element` pointer.
@@ -603,6 +574,11 @@ impl<'le> GeckoElement<'le> {
     }
 
     #[inline(always)]
+    fn get_part_attr(&self) -> Option<&structs::nsAttrValue> {
+        snapshot_helpers::find_attr(self.attrs(), &atom!("part"))
+    }
+
+    #[inline(always)]
     fn get_class_attr(&self) -> Option<&structs::nsAttrValue> {
         if !self.may_have_class() {
             return None;
@@ -894,10 +870,11 @@ impl<'le> GeckoElement<'le> {
         if !self.as_node().is_in_shadow_tree() {
             return false;
         }
-        match self.containing_shadow_host() {
-            Some(e) => e.is_svg_element() && e.local_name() == &*local_name!("use"),
-            None => false,
+        if !self.parent_node_is_shadow_root() {
+            return false;
         }
+        let host = self.containing_shadow_host().unwrap();
+        host.is_svg_element() && host.local_name() == &*local_name!("use")
     }
 
     fn css_transitions_info(&self) -> FxHashMap<LonghandId, Arc<AnimationValue>> {
@@ -1250,14 +1227,6 @@ impl<'le> TElement for GeckoElement<'le> {
         }
     }
 
-    fn has_same_xbl_proto_binding_as(&self, other: Self) -> bool {
-        match (self.xbl_binding(), other.xbl_binding()) {
-            (None, None) => true,
-            (Some(a), Some(b)) => a.0.mPrototypeBinding == b.0.mPrototypeBinding,
-            _ => false,
-        }
-    }
-
     fn each_anonymous_content_child<F>(&self, mut f: F)
     where
         F: FnMut(Self),
@@ -1436,7 +1405,7 @@ impl<'le> TElement for GeckoElement<'le> {
 
     #[inline]
     fn matches_user_and_author_rules(&self) -> bool {
-        !self.is_in_native_anonymous_subtree()
+        !self.rule_hash_target().is_in_native_anonymous_subtree()
     }
 
     #[inline]
@@ -1599,43 +1568,6 @@ impl<'le> TElement for GeckoElement<'le> {
         self.may_have_animations() && unsafe { Gecko_ElementHasCSSTransitions(self.0) }
     }
 
-    fn each_xbl_cascade_data<'a, F>(&self, mut f: F) -> bool
-    where
-        'le: 'a,
-        F: FnMut(&'a CascadeData, QuirksMode),
-    {
-        // Walk the binding scope chain, starting with the binding attached to
-        // our content, up till we run out of scopes or we get cut off.
-        //
-        // If we are a NAC pseudo-element, we want to get rules from our
-        // rule_hash_target, that is, our originating element.
-        let mut current = Some(self.rule_hash_target());
-        while let Some(element) = current {
-            if let Some(binding) = element.xbl_binding() {
-                binding.each_xbl_cascade_data(&mut f);
-
-                // If we're not looking at our original element, allow the
-                // binding to cut off style inheritance.
-                if element != *self && !binding.inherits_style() {
-                    // Go no further; we're not inheriting style from
-                    // anything above here.
-                    break;
-                }
-            }
-
-            if element.is_root_of_native_anonymous_subtree() {
-                // Deliberately cut off style inheritance here.
-                break;
-            }
-
-            current = element.xbl_binding_parent();
-        }
-
-        // If current has something, this means we cut off inheritance at some
-        // point in the loop.
-        current.is_some()
-    }
-
     fn xbl_binding_anonymous_content(&self) -> Option<GeckoNode<'le>> {
         self.xbl_binding_with_content()
             .map(|b| unsafe { GeckoNode::from_content(&*b.anon_content()) })
@@ -2332,6 +2264,16 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
         case_sensitivity.eq_atom(element_id, id)
     }
 
+    #[inline]
+    fn is_part(&self, name: &Atom) -> bool {
+        let attr = match self.get_part_attr() {
+            Some(c) => c,
+            None => return false,
+        };
+
+        snapshot_helpers::has_class_or_part(name, CaseSensitivity::CaseSensitive, attr)
+    }
+
     #[inline(always)]
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         let attr = match self.get_class_attr() {
@@ -2339,7 +2281,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             None => return false,
         };
 
-        snapshot_helpers::has_class(name, case_sensitivity, attr)
+        snapshot_helpers::has_class_or_part(name, case_sensitivity, attr)
     }
 
     #[inline]
diff --git a/components/style/invalidation/element/element_wrapper.rs b/components/style/invalidation/element/element_wrapper.rs
index c794eb15c2c..6e8e504c226 100644
--- a/components/style/invalidation/element/element_wrapper.rs
+++ b/components/style/invalidation/element/element_wrapper.rs
@@ -58,6 +58,10 @@ pub trait ElementSnapshot: Sized {
     /// if `has_attrs()` returns true.
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool;
 
+    /// Whether this snapshot represents the part named `name`. Should only be
+    /// called if `has_attrs()` returns true.
+    fn is_part(&self, name: &Atom) -> bool;
+
     /// A callback that should be called for each class of the snapshot. Should
     /// only be called if `has_attrs()` returns true.
     fn each_class<F>(&self, _: F)
@@ -340,6 +344,13 @@ where
         }
     }
 
+    fn is_part(&self, name: &Atom) -> bool {
+        match self.snapshot() {
+            Some(snapshot) if snapshot.has_attrs() => snapshot.is_part(name),
+            _ => self.element.is_part(name),
+        }
+    }
+
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         match self.snapshot() {
             Some(snapshot) if snapshot.has_attrs() => snapshot.has_class(name, case_sensitivity),
diff --git a/components/style/invalidation/element/invalidation_map.rs b/components/style/invalidation/element/invalidation_map.rs
index e4cc8115760..63d6eb6acc4 100644
--- a/components/style/invalidation/element/invalidation_map.rs
+++ b/components/style/invalidation/element/invalidation_map.rs
@@ -98,6 +98,7 @@ impl Dependency {
             // an eager pseudo, and return only Descendants here if not.
             Some(Combinator::PseudoElement) => DependencyInvalidationKind::ElementAndDescendants,
             Some(Combinator::SlotAssignment) => DependencyInvalidationKind::SlottedElements,
+            Some(Combinator::Part) => unimplemented!("Need to add invalidation for shadow parts"),
         }
     }
 }
diff --git a/components/style/invalidation/element/invalidator.rs b/components/style/invalidation/element/invalidator.rs
index 371c0771183..ec1548f4aa3 100644
--- a/components/style/invalidation/element/invalidator.rs
+++ b/components/style/invalidation/element/invalidator.rs
@@ -158,7 +158,10 @@ impl<'a> Invalidation<'a> {
         // We should be able to do better here!
         match self.selector.combinator_at_parse_order(self.offset - 1) {
             Combinator::Descendant | Combinator::LaterSibling | Combinator::PseudoElement => true,
-            Combinator::SlotAssignment | Combinator::NextSibling | Combinator::Child => false,
+            Combinator::Part |
+            Combinator::SlotAssignment |
+            Combinator::NextSibling |
+            Combinator::Child => false,
         }
     }
 
@@ -171,6 +174,9 @@ impl<'a> Invalidation<'a> {
             Combinator::Child | Combinator::Descendant | Combinator::PseudoElement => {
                 InvalidationKind::Descendant(DescendantInvalidationKind::Dom)
             },
+            Combinator::Part => {
+                unimplemented!("Need to add invalidation for shadow parts");
+            },
             Combinator::SlotAssignment => {
                 InvalidationKind::Descendant(DescendantInvalidationKind::Slotted)
             },
diff --git a/components/style/invalidation/element/state_and_attributes.rs b/components/style/invalidation/element/state_and_attributes.rs
index e4555bb9a70..a49bb6306c5 100644
--- a/components/style/invalidation/element/state_and_attributes.rs
+++ b/components/style/invalidation/element/state_and_attributes.rs
@@ -224,8 +224,8 @@ where
 
         let mut shadow_rule_datas = SmallVec::<[_; 3]>::new();
         let matches_document_author_rules =
-            element.each_applicable_non_document_style_rule_data(|data, quirks_mode, host| {
-                shadow_rule_datas.push((data, quirks_mode, host.map(|h| h.opaque())))
+            element.each_applicable_non_document_style_rule_data(|data, host| {
+                shadow_rule_datas.push((data, host.opaque()))
             });
 
         let invalidated_self = {
@@ -258,12 +258,8 @@ where
                 }
             }
 
-            for &(ref data, quirks_mode, ref host) in &shadow_rule_datas {
-                // FIXME(emilio): Replace with assert / remove when we figure
-                // out what to do with the quirks mode mismatches
-                // (that is, when bug 1406875 is properly fixed).
-                collector.matching_context.set_quirks_mode(quirks_mode);
-                collector.matching_context.current_host = host.clone();
+            for &(ref data, ref host) in &shadow_rule_datas {
+                collector.matching_context.current_host = Some(host.clone());
                 collector.collect_dependencies_in_invalidation_map(data.invalidation_map());
             }
 
diff --git a/components/style/properties/data.py b/components/style/properties/data.py
index 407cc5fcdb0..7a96402c4a1 100644
--- a/components/style/properties/data.py
+++ b/components/style/properties/data.py
@@ -347,6 +347,7 @@ class Longhand(object):
                 "TextAlign",
                 "TextDecorationLine",
                 "TextEmphasisPosition",
+                "TextTransform",
                 "TouchAction",
                 "TransformStyle",
                 "UserSelect",
@@ -399,8 +400,6 @@ class Shorthand(object):
             and allowed_in_keyframe_block != "False"
 
     def get_animatable(self):
-        if self.ident == "all":
-            return False
         for sub in self.sub_properties:
             if sub.animatable:
                 return True
diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index 81e1ee4307d..b73bb073fa6 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -38,7 +38,7 @@ use crate::gecko_bindings::bindings::{Gecko_ResetFilters, Gecko_CopyFiltersFrom}
 use crate::gecko_bindings::structs;
 use crate::gecko_bindings::structs::nsCSSPropertyID;
 use crate::gecko_bindings::structs::mozilla::PseudoStyleType;
-use crate::gecko_bindings::sugar::ns_style_coord::{CoordDataValue, CoordData, CoordDataMut};
+use crate::gecko_bindings::sugar::ns_style_coord::{CoordDataValue, CoordDataMut};
 use crate::gecko_bindings::sugar::refptr::RefPtr;
 use crate::gecko::values::GeckoStyleCoordConvertible;
 use crate::gecko::values::round_border_to_device_pixels;
@@ -1104,9 +1104,6 @@ impl ${style_struct.gecko_struct_name} {
         }
         result
     }
-    pub fn get_gecko(&self) -> &${style_struct.gecko_ffi_name} {
-        &self.gecko
-    }
 }
 impl Drop for ${style_struct.gecko_struct_name} {
     fn drop(&mut self) {
@@ -2508,7 +2505,7 @@ fn static_assert() {
     }
 </%def>
 
-<% skip_box_longhands= """display vertical-align
+<% skip_box_longhands= """display
                           animation-name animation-delay animation-duration
                           animation-direction animation-fill-mode animation-play-state
                           animation-iteration-count animation-timing-function
@@ -2564,47 +2561,6 @@ fn static_assert() {
     ) %>
     ${impl_keyword('clear', 'mBreakType', clear_keyword)}
 
-    pub fn set_vertical_align(&mut self, v: longhands::vertical_align::computed_value::T) {
-        use crate::values::generics::box_::VerticalAlign;
-        let value = match v {
-            VerticalAlign::Baseline => structs::NS_STYLE_VERTICAL_ALIGN_BASELINE,
-            VerticalAlign::Sub => structs::NS_STYLE_VERTICAL_ALIGN_SUB,
-            VerticalAlign::Super => structs::NS_STYLE_VERTICAL_ALIGN_SUPER,
-            VerticalAlign::Top => structs::NS_STYLE_VERTICAL_ALIGN_TOP,
-            VerticalAlign::TextTop => structs::NS_STYLE_VERTICAL_ALIGN_TEXT_TOP,
-            VerticalAlign::Middle => structs::NS_STYLE_VERTICAL_ALIGN_MIDDLE,
-            VerticalAlign::Bottom => structs::NS_STYLE_VERTICAL_ALIGN_BOTTOM,
-            VerticalAlign::TextBottom => structs::NS_STYLE_VERTICAL_ALIGN_TEXT_BOTTOM,
-            VerticalAlign::MozMiddleWithBaseline => {
-                structs::NS_STYLE_VERTICAL_ALIGN_MIDDLE_WITH_BASELINE
-            },
-            VerticalAlign::Length(length) => {
-                self.gecko.mVerticalAlign.set(length);
-                return;
-            },
-        };
-        self.gecko.mVerticalAlign.set_value(CoordDataValue::Enumerated(value));
-    }
-
-    pub fn clone_vertical_align(&self) -> longhands::vertical_align::computed_value::T {
-        use crate::values::computed::LengthPercentage;
-        use crate::values::generics::box_::VerticalAlign;
-
-        let gecko = &self.gecko.mVerticalAlign;
-        match gecko.as_value() {
-            CoordDataValue::Enumerated(value) => VerticalAlign::from_gecko_keyword(value),
-            _ => {
-                VerticalAlign::Length(
-                    LengthPercentage::from_gecko_style_coord(gecko).expect(
-                        "expected <length-percentage> for vertical-align",
-                    ),
-                )
-            },
-        }
-    }
-
-    <%call expr="impl_coord_copy('vertical_align', 'mVerticalAlign')"></%call>
-
     ${impl_style_coord("scroll_snap_points_x", "mScrollSnapPointsX")}
     ${impl_style_coord("scroll_snap_points_y", "mScrollSnapPointsY")}
 
@@ -4455,7 +4411,7 @@ clip-path
         fn set_counter_function(
             data: &mut nsStyleContentData,
             content_type: StyleContentType,
-            name: &CustomIdent,
+            name: CustomIdent,
             sep: &str,
             style: CounterStyleOrNone,
         ) {
@@ -4464,7 +4420,9 @@ clip-path
             let counter_func = unsafe {
                 bindings::Gecko_SetCounterFunction(data, content_type).as_mut().unwrap()
             };
-            counter_func.mIdent.assign(name.0.as_slice());
+            counter_func.mIdent.set_move(unsafe {
+                RefPtr::from_addrefed(name.0.into_addrefed())
+            });
             if content_type == StyleContentType::Counters {
                 counter_func.mSeparator.assign_str(sep);
             }
@@ -4493,14 +4451,14 @@ clip-path
                     Gecko_ClearAndResizeStyleContents(&mut self.gecko,
                                                       items.len() as u32);
                 }
-                for (i, item) in items.into_iter().enumerate() {
+                for (i, item) in items.into_vec().into_iter().enumerate() {
                     // NB: Gecko compares the mString value if type is not image
                     // or URI independently of whatever gets there. In the quote
                     // cases, they set it to null, so do the same here.
                     unsafe {
                         *self.gecko.mContents[i].mContent.mString.as_mut() = ptr::null_mut();
                     }
-                    match *item {
+                    match item {
                         ContentItem::String(ref value) => {
                             self.gecko.mContents[i].mType = StyleContentType::String;
                             unsafe {
@@ -4536,22 +4494,22 @@ clip-path
                             => self.gecko.mContents[i].mType = StyleContentType::NoOpenQuote,
                         ContentItem::NoCloseQuote
                             => self.gecko.mContents[i].mType = StyleContentType::NoCloseQuote,
-                        ContentItem::Counter(ref name, ref style) => {
+                        ContentItem::Counter(name, style) => {
                             set_counter_function(
                                 &mut self.gecko.mContents[i],
                                 StyleContentType::Counter,
-                                &name,
+                                name,
                                 "",
-                                style.clone(),
+                                style,
                             );
                         }
-                        ContentItem::Counters(ref name, ref sep, ref style) => {
+                        ContentItem::Counters(name, sep, style) => {
                             set_counter_function(
                                 &mut self.gecko.mContents[i],
                                 StyleContentType::Counters,
-                                &name,
+                                name,
                                 &sep,
-                                style.clone(),
+                                style,
                             );
                         }
                         ContentItem::Url(ref url) => {
@@ -4627,7 +4585,9 @@ clip-path
                     StyleContentType::Counter | StyleContentType::Counters => {
                         let gecko_function =
                             unsafe { &**gecko_content.mContent.mCounters.as_ref() };
-                        let ident = CustomIdent(Atom::from(&*gecko_function.mIdent));
+                        let ident = CustomIdent(unsafe {
+                            Atom::from_raw(gecko_function.mIdent.mRawPtr)
+                        });
                         let style =
                             CounterStyleOrNone::from_gecko_value(&gecko_function.mCounterStyle);
                         let style = match style {
@@ -4664,8 +4624,10 @@ clip-path
         ) {
             unsafe {
                 bindings::Gecko_ClearAndResizeCounter${counter_property}s(&mut self.gecko, v.len() as u32);
-                for (i, ref pair) in v.iter().enumerate() {
-                    self.gecko.m${counter_property}s[i].mCounter.assign(pair.name.0.as_slice());
+                for (i, pair) in v.0.into_vec().into_iter().enumerate() {
+                    self.gecko.m${counter_property}s[i].mCounter.set_move(
+                        RefPtr::from_addrefed(pair.name.0.into_addrefed())
+                    );
                     self.gecko.m${counter_property}s[i].mValue = pair.value;
                 }
             }
@@ -4690,7 +4652,9 @@ clip-path
             longhands::counter_${counter_property.lower()}::computed_value::T::new(
                 self.gecko.m${counter_property}s.iter().map(|ref gecko_counter| {
                     CounterPair {
-                        name: CustomIdent(Atom::from(gecko_counter.mCounter.to_string())),
+                        name: CustomIdent(unsafe {
+                            Atom::from_raw(gecko_counter.mCounter.mRawPtr)
+                        }),
                         value: gecko_counter.mValue,
                     }
                 }).collect()
diff --git a/components/style/properties/longhands/box.mako.rs b/components/style/properties/longhands/box.mako.rs
index 397393ed3d8..a7a2cb9e348 100644
--- a/components/style/properties/longhands/box.mako.rs
+++ b/components/style/properties/longhands/box.mako.rs
@@ -337,11 +337,11 @@ ${helpers.predefined_type(
     "Position",
     "computed::Position::zero()",
     vector=True,
+    allow_empty=True,
     products="gecko",
     gecko_pref="layout.css.scroll-snap.enabled",
     spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/scroll-snap-destination)",
     animation_value_type="discrete",
-    allow_empty="NotInitial",
 )}
 
 <% transform_extra_prefixes = "moz:layout.css.prefixes.transforms webkit" %>
diff --git a/components/style/properties/longhands/inherited_table.mako.rs b/components/style/properties/longhands/inherited_table.mako.rs
index 7dc4d73fe62..246a073d2a8 100644
--- a/components/style/properties/longhands/inherited_table.mako.rs
+++ b/components/style/properties/longhands/inherited_table.mako.rs
@@ -9,7 +9,7 @@
 ${helpers.single_keyword(
     "border-collapse",
     "separate collapse",
-    gecko_constant_prefix="NS_STYLE_BORDER",
+    gecko_enum_prefix="StyleBorderCollapse",
     animation_value_type="discrete",
     spec="https://drafts.csswg.org/css-tables/#propdef-border-collapse",
     servo_restyle_damage = "reflow",
diff --git a/components/style/properties/longhands/inherited_text.mako.rs b/components/style/properties/longhands/inherited_text.mako.rs
index f61c8754b5e..cdd7df93256 100644
--- a/components/style/properties/longhands/inherited_text.mako.rs
+++ b/components/style/properties/longhands/inherited_text.mako.rs
@@ -19,11 +19,10 @@ ${helpers.predefined_type(
 
 // CSS Text Module Level 3
 
-// TODO(pcwalton): `full-width`
-${helpers.single_keyword(
+${helpers.predefined_type(
     "text-transform",
-    "none capitalize uppercase lowercase",
-    extra_gecko_values="full-width full-size-kana",
+    "TextTransform",
+    "computed::TextTransform::none()",
     animation_value_type="discrete",
     flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
     spec="https://drafts.csswg.org/css-text/#propdef-text-transform",
diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 5f44a259641..e7f9c72413e 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -27,7 +27,7 @@ use crate::context::QuirksMode;
 #[cfg(feature = "servo")] use crate::logical_geometry::LogicalMargin;
 #[cfg(feature = "servo")] use crate::computed_values;
 use crate::logical_geometry::WritingMode;
-#[cfg(feature = "gecko")] use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use crate::media_queries::Device;
 use crate::parser::ParserContext;
 use crate::properties::longhands::system_font::SystemFont;
@@ -376,7 +376,6 @@ impl PartialEq for PropertyDeclaration {
     }
 }
 
-#[cfg(feature = "gecko")]
 impl MallocSizeOf for PropertyDeclaration {
     #[inline]
     fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
@@ -1086,6 +1085,8 @@ impl LonghandId {
         }
     }
 
+    // TODO(emilio): Should we use a function table like CASCADE_PROPERTY does
+    // to avoid blowing up code-size here?
     fn parse_value<'i, 't>(
         &self,
         context: &ParserContext,
@@ -1533,64 +1534,7 @@ impl UnparsedValue {
         quirks_mode: QuirksMode,
         environment: &::custom_properties::CssEnvironment,
     ) -> PropertyDeclaration {
-        crate::custom_properties::substitute(
-            &self.css,
-            self.first_token_type,
-            custom_properties,
-            environment,
-        ).ok().and_then(|css| {
-            // As of this writing, only the base URL is used for property
-            // values.
-            //
-            // NOTE(emilio): we intentionally pase `None` as the rule type here.
-            // If something starts depending on it, it's probably a bug, since
-            // it'd change how values are parsed depending on whether we're in a
-            // @keyframes rule or not, for example... So think twice about
-            // whether you want to do this!
-            //
-            // FIXME(emilio): ParsingMode is slightly fishy...
-            let context = ParserContext::new(
-                Origin::Author,
-                &self.url_data,
-                None,
-                ParsingMode::DEFAULT,
-                quirks_mode,
-                None,
-                None,
-            );
-
-            let mut input = ParserInput::new(&css);
-            Parser::new(&mut input).parse_entirely(|input| {
-                match self.from_shorthand {
-                    None => longhand_id.parse_value(&context, input),
-                    Some(ShorthandId::All) => {
-                        // No need to parse the 'all' shorthand as anything other than a CSS-wide
-                        // keyword, after variable substitution.
-                        Err(input.new_custom_error(SelectorParseErrorKind::UnexpectedIdent("all".into())))
-                    }
-                    % for shorthand in data.shorthands_except_all():
-                        Some(ShorthandId::${shorthand.camel_case}) => {
-                            shorthands::${shorthand.ident}::parse_value(&context, input)
-                            .map(|longhands| {
-                                match longhand_id {
-                                    % for property in shorthand.sub_properties:
-                                        LonghandId::${property.camel_case} => {
-                                            PropertyDeclaration::${property.camel_case}(
-                                                longhands.${property.ident}
-                                            )
-                                        }
-                                    % endfor
-                                    _ => unreachable!()
-                                }
-                            })
-                        }
-                    % endfor
-                }
-            })
-            .ok()
-        })
-        .unwrap_or_else(|| {
-            // Invalid at computed-value time.
+        let invalid_at_computed_value_time = || {
             let keyword = if longhand_id.inherited() {
                 CSSWideKeyword::Inherit
             } else {
@@ -1600,7 +1544,80 @@ impl UnparsedValue {
                 id: longhand_id,
                 keyword,
             })
-        })
+        };
+
+        let css = match crate::custom_properties::substitute(
+            &self.css,
+            self.first_token_type,
+            custom_properties,
+            environment,
+        ) {
+            Ok(css) => css,
+            Err(..) => return invalid_at_computed_value_time(),
+        };
+
+        // As of this writing, only the base URL is used for property
+        // values.
+        //
+        // NOTE(emilio): we intentionally pase `None` as the rule type here.
+        // If something starts depending on it, it's probably a bug, since
+        // it'd change how values are parsed depending on whether we're in a
+        // @keyframes rule or not, for example... So think twice about
+        // whether you want to do this!
+        //
+        // FIXME(emilio): ParsingMode is slightly fishy...
+        let context = ParserContext::new(
+            Origin::Author,
+            &self.url_data,
+            None,
+            ParsingMode::DEFAULT,
+            quirks_mode,
+            None,
+            None,
+        );
+
+        let mut input = ParserInput::new(&css);
+        let mut input = Parser::new(&mut input);
+        input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
+        if let Ok(keyword) = input.try(CSSWideKeyword::parse) {
+            return PropertyDeclaration::CSSWideKeyword(WideKeywordDeclaration {
+                id: longhand_id,
+                keyword,
+            });
+        }
+
+        let declaration = input.parse_entirely(|input| {
+            match self.from_shorthand {
+                None => longhand_id.parse_value(&context, input),
+                Some(ShorthandId::All) => {
+                    // No need to parse the 'all' shorthand as anything other
+                    // than a CSS-wide keyword, after variable substitution.
+                    Err(input.new_custom_error(SelectorParseErrorKind::UnexpectedIdent("all".into())))
+                }
+                % for shorthand in data.shorthands_except_all():
+                Some(ShorthandId::${shorthand.camel_case}) => {
+                    shorthands::${shorthand.ident}::parse_value(&context, input)
+                    .map(|longhands| {
+                        match longhand_id {
+                            % for property in shorthand.sub_properties:
+                                LonghandId::${property.camel_case} => {
+                                    PropertyDeclaration::${property.camel_case}(
+                                        longhands.${property.ident}
+                                    )
+                                }
+                            % endfor
+                            _ => unreachable!()
+                        }
+                    })
+                }
+                % endfor
+            }
+        });
+
+        match declaration {
+            Ok(decl) => decl,
+            Err(..) => invalid_at_computed_value_time(),
+        }
     }
 }
 
@@ -3745,8 +3762,7 @@ pub fn adjust_border_width(style: &mut StyleBuilder) {
 }
 
 /// An identifier for a given alias property.
-#[derive(Clone, Copy, Eq, PartialEq)]
-#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+#[derive(Clone, Copy, Eq, PartialEq, MallocSizeOf)]
 #[repr(u16)]
 pub enum AliasId {
     % for i, property in enumerate(data.all_aliases()):
diff --git a/components/style/properties/shorthands/position.mako.rs b/components/style/properties/shorthands/position.mako.rs
index aaf97a0674c..6fdbe1235cf 100644
--- a/components/style/properties/shorthands/position.mako.rs
+++ b/components/style/properties/shorthands/position.mako.rs
@@ -254,7 +254,7 @@
     use crate::parser::Parse;
     use servo_arc::Arc;
     use crate::values::{Either, None_};
-    use crate::values::generics::grid::{LineNameList, TrackSize, TrackList, TrackListType};
+    use crate::values::generics::grid::{TrackSize, TrackList, TrackListType};
     use crate::values::generics::grid::{TrackListValue, concat_serialize_idents};
     use crate::values::specified::{GridTemplateComponent, GenericGridTemplateComponent};
     use crate::values::specified::grid::parse_line_names;
@@ -265,12 +265,11 @@
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
     ) -> Result<(GridTemplateComponent, GridTemplateComponent, Either<TemplateAreasArc, None_>), ParseError<'i>> {
-        // Other shorthand sub properties also parse `none` and `subgrid` keywords and this
-        // shorthand should know after these keywords there is nothing to parse. Otherwise it
-        // gets confused and rejects the sub properties that contains `none` or `subgrid`.
+        // Other shorthand sub properties also parse the `none` keyword and this shorthand
+        // should know after this keyword there is nothing to parse. Otherwise it gets
+        // confused and rejects the sub properties that contains `none`.
         <% keywords = {
             "none": "GenericGridTemplateComponent::None",
-            "subgrid": "GenericGridTemplateComponent::Subgrid(LineNameList::default())"
         }
         %>
         % for keyword, rust_type in keywords.items():
diff --git a/components/style/rule_collector.rs b/components/style/rule_collector.rs
index ba602ddca83..e87a86c5e47 100644
--- a/components/style/rule_collector.rs
+++ b/components/style/rule_collector.rs
@@ -98,7 +98,7 @@ where
         flags_setter: &'a mut F,
     ) -> Self {
         let rule_hash_target = element.rule_hash_target();
-        let matches_user_and_author_rules = rule_hash_target.matches_user_and_author_rules();
+        let matches_user_and_author_rules = element.matches_user_and_author_rules();
 
         // Gecko definitely has pseudo-elements with style attributes, like
         // ::-moz-color-swatch.
@@ -198,7 +198,7 @@ where
         let rules = &mut self.rules;
         let flags_setter = &mut self.flags_setter;
         let shadow_cascade_order = self.shadow_cascade_order;
-        self.context.with_shadow_host(Some(shadow_host), |context| {
+        self.context.with_shadow_host(shadow_host, |context| {
             map.get_all_matching_rules(
                 element,
                 rule_hash_target,
@@ -303,42 +303,6 @@ where
         self.collect_stylist_rules(Origin::Author);
     }
 
-    fn collect_xbl_rules(&mut self) {
-        let element = self.element;
-        let cut_xbl_binding_inheritance =
-            element.each_xbl_cascade_data(|cascade_data, quirks_mode| {
-                let map = match cascade_data.normal_rules(self.pseudo_element) {
-                    Some(m) => m,
-                    None => return,
-                };
-
-                // NOTE(emilio): This is needed because the XBL stylist may
-                // think it has a different quirks mode than the document.
-                let mut matching_context = MatchingContext::new(
-                    self.context.matching_mode(),
-                    self.context.bloom_filter,
-                    self.context.nth_index_cache.as_mut().map(|s| &mut **s),
-                    quirks_mode,
-                );
-                matching_context.pseudo_element_matching_fn =
-                    self.context.pseudo_element_matching_fn;
-
-                // SameTreeAuthorNormal instead of InnerShadowNormal to
-                // preserve behavior, though that's kinda fishy...
-                map.get_all_matching_rules(
-                    self.element,
-                    self.rule_hash_target,
-                    self.rules,
-                    &mut matching_context,
-                    self.flags_setter,
-                    CascadeLevel::SameTreeAuthorNormal,
-                    self.shadow_cascade_order,
-                );
-            });
-
-        self.matches_document_author_rules &= !cut_xbl_binding_inheritance;
-    }
-
     fn collect_style_attribute_and_animation_rules(&mut self) {
         if let Some(sa) = self.style_attribute {
             self.rules
@@ -396,7 +360,6 @@ where
         self.collect_host_rules();
         self.collect_slotted_rules();
         self.collect_normal_rules_from_containing_shadow_tree();
-        self.collect_xbl_rules();
         self.collect_document_author_rules();
         self.collect_style_attribute_and_animation_rules();
     }
diff --git a/components/style/servo/selector_parser.rs b/components/style/servo/selector_parser.rs
index 9d94c467b58..1ef49c87f9f 100644
--- a/components/style/servo/selector_parser.rs
+++ b/components/style/servo/selector_parser.rs
@@ -66,10 +66,6 @@ pub const PSEUDO_COUNT: usize = PseudoElement::ServoInlineAbsolute as usize + 1;
 
 impl ::selectors::parser::PseudoElement for PseudoElement {
     type Impl = SelectorImpl;
-
-    fn supports_pseudo_class(&self, _: &NonTSPseudoClass) -> bool {
-        false
-    }
 }
 
 impl ToCss for PseudoElement {
@@ -293,6 +289,14 @@ impl ::selectors::parser::NonTSPseudoClass for NonTSPseudoClass {
     fn is_active_or_hover(&self) -> bool {
         matches!(*self, NonTSPseudoClass::Active | NonTSPseudoClass::Hover)
     }
+
+    #[inline]
+    fn is_user_action_state(&self) -> bool {
+        matches!(
+            *self,
+            NonTSPseudoClass::Active | NonTSPseudoClass::Hover | NonTSPseudoClass::Focus
+        )
+    }
 }
 
 impl ToCss for NonTSPseudoClass {
@@ -393,6 +397,7 @@ impl ::selectors::SelectorImpl for SelectorImpl {
     type AttrValue = String;
     type Identifier = Atom;
     type ClassName = Atom;
+    type PartName = Atom;
     type LocalName = LocalName;
     type NamespacePrefix = Prefix;
     type NamespaceUrl = Namespace;
@@ -679,6 +684,10 @@ impl ElementSnapshot for ServoElementSnapshot {
             .map(|v| v.as_atom())
     }
 
+    fn is_part(&self, _name: &Atom) -> bool {
+        false
+    }
+
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         self.get_attr(&ns!(), &local_name!("class"))
             .map_or(false, |v| {
diff --git a/components/style/sharing/mod.rs b/components/style/sharing/mod.rs
index 0675359ed77..3ce2a717dc8 100644
--- a/components/style/sharing/mod.rs
+++ b/components/style/sharing/mod.rs
@@ -727,27 +727,6 @@ impl<E: TElement> StyleSharingCache<E> {
             return None;
         }
 
-        // Note that in theory we shouldn't need this XBL check. However, XBL is
-        // absolutely broken in all sorts of ways.
-        //
-        // A style change that changes which XBL binding applies to an element
-        // arrives there, with the element still having the old prototype
-        // binding attached. And thus we try to match revalidation selectors
-        // with the old XBL binding, because we can't look at the new ones of
-        // course. And that causes us to revalidate with the wrong selectors and
-        // hit assertions.
-        //
-        // Other than this, we don't need anything else like the containing XBL
-        // binding parent or what not, since two elements with different XBL
-        // bindings will necessarily end up with different style.
-        if !target
-            .element
-            .has_same_xbl_proto_binding_as(candidate.element)
-        {
-            trace!("Miss: Different proto bindings");
-            return None;
-        }
-
         // If the elements are not assigned to the same slot they could match
         // different ::slotted() rules in the slot scope.
         //
diff --git a/components/style/stylesheet_set.rs b/components/style/stylesheet_set.rs
index d2c3ad0f88e..d8c21d5494c 100644
--- a/components/style/stylesheet_set.rs
+++ b/components/style/stylesheet_set.rs
@@ -468,7 +468,14 @@ where
             .fold(0, |s, (item, _)| s + item.len())
     }
 
+    /// Returns the count of stylesheets for a given origin.
+    #[inline]
+    pub fn sheet_count(&self, origin: Origin) -> usize {
+        self.collections.borrow_for_origin(&origin).len()
+    }
+
     /// Returns the `index`th stylesheet in the set for the given origin.
+    #[inline]
     pub fn get(&self, origin: Origin, index: usize) -> Option<&S> {
         self.collections.borrow_for_origin(&origin).get(index)
     }
@@ -539,7 +546,7 @@ where
     }
 }
 
-/// The set of stylesheets effective for a given XBL binding or Shadow Root.
+/// The set of stylesheets effective for a given Shadow Root.
 #[derive(MallocSizeOf)]
 pub struct AuthorStylesheetSet<S>
 where
diff --git a/components/style/stylesheets/origin.rs b/components/style/stylesheets/origin.rs
index 783b8f26a8b..a65b61fca13 100644
--- a/components/style/stylesheets/origin.rs
+++ b/components/style/stylesheets/origin.rs
@@ -10,18 +10,17 @@ use std::ops::BitOrAssign;
 /// Each style rule has an origin, which determines where it enters the cascade.
 ///
 /// <https://drafts.csswg.org/css-cascade/#cascading-origins>
-#[derive(Clone, Copy, Debug, Eq, PartialEq, ToShmem)]
+#[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq, ToShmem)]
 #[repr(u8)]
-#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub enum Origin {
     /// <https://drafts.csswg.org/css-cascade/#cascade-origin-user-agent>
-    UserAgent = 1 << 0,
+    UserAgent = 0x1,
 
     /// <https://drafts.csswg.org/css-cascade/#cascade-origin-user>
-    User = 1 << 1,
+    User = 0x2,
 
     /// <https://drafts.csswg.org/css-cascade/#cascade-origin-author>
-    Author = 1 << 2,
+    Author = 0x4,
 }
 
 impl Origin {
@@ -59,7 +58,7 @@ impl Origin {
 
 bitflags! {
     /// A set of origins. This is equivalent to Gecko's OriginFlags.
-    #[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+    #[derive(MallocSizeOf)]
     pub struct OriginSet: u8 {
         /// <https://drafts.csswg.org/css-cascade/#cascade-origin-user-agent>
         const ORIGIN_USER_AGENT = Origin::UserAgent as u8;
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index ece14e9896f..2953e3f0671 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -48,8 +48,9 @@ use selectors::visitor::SelectorVisitor;
 use selectors::NthIndexCache;
 use servo_arc::{Arc, ArcBorrow};
 use smallbitvec::SmallBitVec;
-use std::ops;
+use smallvec::SmallVec;
 use std::sync::Mutex;
+use std::{mem, ops};
 use style_traits::viewport::ViewportConstraints;
 
 /// The type of the stylesheets that the stylist contains.
@@ -128,15 +129,28 @@ impl UserAgentCascadeDataCache {
         Ok(new_data)
     }
 
-    fn expire_unused(&mut self) {
-        // is_unique() returns false for static references, but we never have
-        // static references to UserAgentCascadeDatas.  If we did, it may not
-        // make sense to put them in the cache in the first place.
-        self.entries.retain(|e| !e.is_unique())
+    /// Returns all the cascade datas that are not being used (that is, that are
+    /// held alive just by this cache).
+    ///
+    /// We return them instead of dropping in place because some of them may
+    /// keep alive some other documents (like the SVG documents kept alive by
+    /// URL references), and thus we don't want to drop them while locking the
+    /// cache to not deadlock.
+    fn take_unused(&mut self) -> SmallVec<[Arc<UserAgentCascadeData>; 3]> {
+        let mut unused = SmallVec::new();
+        for i in (0..self.entries.len()).rev() {
+            // is_unique() returns false for static references, but we never
+            // have static references to UserAgentCascadeDatas.  If we did, it
+            // may not make sense to put them in the cache in the first place.
+            if self.entries[i].is_unique() {
+                unused.push(self.entries.remove(i));
+            }
+        }
+        unused
     }
 
-    fn clear(&mut self) {
-        self.entries.clear();
+    fn take_all(&mut self) -> Vec<Arc<UserAgentCascadeData>> {
+        mem::replace(&mut self.entries, Vec::new())
     }
 
     #[cfg(feature = "gecko")]
@@ -254,13 +268,14 @@ impl DocumentCascadeData {
         // First do UA sheets.
         {
             if flusher.flush_origin(Origin::UserAgent).dirty() {
-                let mut ua_cache = UA_CASCADE_DATA_CACHE.lock().unwrap();
                 let origin_sheets = flusher.origin_sheets(Origin::UserAgent);
-                let ua_cascade_data =
-                    ua_cache.lookup(origin_sheets, device, quirks_mode, guards.ua_or_user)?;
-                ua_cache.expire_unused();
-                debug!("User agent data cache size {:?}", ua_cache.len());
-                self.user_agent = ua_cascade_data;
+                let _unused_cascade_datas = {
+                    let mut ua_cache = UA_CASCADE_DATA_CACHE.lock().unwrap();
+                    self.user_agent =
+                        ua_cache.lookup(origin_sheets, device, quirks_mode, guards.ua_or_user)?;
+                    debug!("User agent data cache size {:?}", ua_cache.len());
+                    ua_cache.take_unused()
+                };
             }
         }
 
@@ -591,6 +606,18 @@ impl Stylist {
             .remove_stylesheet(Some(&self.device), sheet, guard)
     }
 
+    /// Appends a new stylesheet to the current set.
+    #[inline]
+    pub fn sheet_count(&self, origin: Origin) -> usize {
+        self.stylesheets.sheet_count(origin)
+    }
+
+    /// Appends a new stylesheet to the current set.
+    #[inline]
+    pub fn sheet_at(&self, origin: Origin, index: usize) -> Option<&StylistSheet> {
+        self.stylesheets.get(origin, index)
+    }
+
     /// Returns whether for any of the applicable style rule data a given
     /// condition is true.
     pub fn any_applicable_rule_data<E, F>(&self, element: E, mut f: F) -> bool
@@ -605,7 +632,7 @@ impl Stylist {
         let mut maybe = false;
 
         let doc_author_rules_apply =
-            element.each_applicable_non_document_style_rule_data(|data, _, _| {
+            element.each_applicable_non_document_style_rule_data(|data, _| {
                 maybe = maybe || f(&*data);
             });
 
@@ -1041,12 +1068,6 @@ impl Stylist {
     /// Returns whether, given a media feature change, any previously-applicable
     /// style has become non-applicable, or vice-versa for each origin, using
     /// `device`.
-    ///
-    /// Passing `device` is needed because this is used for XBL in Gecko, which
-    /// can be stale in various ways, so we need to pass the device of the
-    /// document itself, which is what is kept up-to-date.
-    ///
-    /// Arguably XBL should use something more lightweight than a Stylist.
     pub fn media_features_change_changed_style(
         &self,
         guards: &StylesheetGuards,
@@ -1230,11 +1251,11 @@ impl Stylist {
         let mut results = SmallBitVec::new();
 
         let matches_document_rules =
-            element.each_applicable_non_document_style_rule_data(|data, quirks_mode, host| {
+            element.each_applicable_non_document_style_rule_data(|data, host| {
                 matching_context.with_shadow_host(host, |matching_context| {
                     data.selectors_for_cache_revalidation.lookup(
                         element,
-                        quirks_mode,
+                        self.quirks_mode,
                         |selector_and_hashes| {
                             results.push(matches_selector(
                                 &selector_and_hashes.selector,
@@ -1356,7 +1377,7 @@ impl Stylist {
 
     /// Shutdown the static data that this module stores.
     pub fn shutdown() {
-        UA_CASCADE_DATA_CACHE.lock().unwrap().clear()
+        let _entries = UA_CASCADE_DATA_CACHE.lock().unwrap().take_all();
     }
 }
 
diff --git a/components/style/values/computed/image.rs b/components/style/values/computed/image.rs
index 4070e51815a..70ecad6a04e 100644
--- a/components/style/values/computed/image.rs
+++ b/components/style/values/computed/image.rs
@@ -55,7 +55,7 @@ pub enum LineDirection {
 pub type EndingShape = generic::EndingShape<Length, LengthPercentage>;
 
 /// A computed gradient item.
-pub type GradientItem = generic::GradientItem<Color, LengthPercentage>;
+pub type GradientItem = generic::GenericGradientItem<Color, LengthPercentage>;
 
 /// A computed color stop.
 pub type ColorStop = generic::ColorStop<Color, LengthPercentage>;
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index 3b32f8fecd7..3eb0c16836c 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -86,6 +86,7 @@ pub use self::transform::{TransformOrigin, TransformStyle, Translate};
 #[cfg(feature = "gecko")]
 pub use self::ui::CursorImage;
 pub use self::ui::{Cursor, MozForceBrokenImageIcon, UserSelect};
+pub use super::specified::TextTransform;
 pub use super::specified::{BorderStyle, TextDecorationLine};
 pub use super::{Auto, Either, None_};
 pub use app_units::Au;
diff --git a/components/style/values/computed/text.rs b/components/style/values/computed/text.rs
index 2fdb1e48c4f..c29d3d45210 100644
--- a/components/style/values/computed/text.rs
+++ b/components/style/values/computed/text.rs
@@ -19,6 +19,7 @@ use std::fmt::{self, Write};
 use style_traits::{CssWriter, ToCss};
 
 pub use crate::values::specified::TextAlignKeyword as TextAlign;
+pub use crate::values::specified::TextTransform;
 pub use crate::values::specified::{OverflowWrap, WordBreak};
 pub use crate::values::specified::{TextDecorationLine, TextEmphasisPosition};
 
diff --git a/components/style/values/generics/box.rs b/components/style/values/generics/box.rs
index 3e8f959f456..5d6e8e25031 100644
--- a/components/style/values/generics/box.rs
+++ b/components/style/values/generics/box.rs
@@ -6,14 +6,15 @@
 
 use crate::values::animated::ToAnimatedZero;
 
-/// A generic value for the `vertical-align` property.
 #[derive(
     Animate,
     Clone,
     ComputeSquaredDistance,
     Copy,
     Debug,
+    FromPrimitive,
     MallocSizeOf,
+    Parse,
     PartialEq,
     SpecifiedValueInfo,
     ToComputedValue,
@@ -21,35 +22,51 @@ use crate::values::animated::ToAnimatedZero;
     ToResolvedValue,
     ToShmem,
 )]
-pub enum VerticalAlign<LengthPercentage> {
-    /// `baseline`
+#[repr(u8)]
+#[allow(missing_docs)]
+pub enum VerticalAlignKeyword {
     Baseline,
-    /// `sub`
     Sub,
-    /// `super`
     Super,
-    /// `top`
     Top,
-    /// `text-top`
     TextTop,
-    /// `middle`
     Middle,
-    /// `bottom`
     Bottom,
-    /// `text-bottom`
     TextBottom,
-    /// `-moz-middle-with-baseline`
     #[cfg(feature = "gecko")]
     MozMiddleWithBaseline,
+}
+
+/// A generic value for the `vertical-align` property.
+#[derive(
+    Animate,
+    Clone,
+    ComputeSquaredDistance,
+    Copy,
+    Debug,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum GenericVerticalAlign<LengthPercentage> {
+    /// One of the vertical-align keywords.
+    Keyword(VerticalAlignKeyword),
     /// `<length-percentage>`
     Length(LengthPercentage),
 }
 
+pub use self::GenericVerticalAlign as VerticalAlign;
+
 impl<L> VerticalAlign<L> {
     /// Returns `baseline`.
     #[inline]
     pub fn baseline() -> Self {
-        VerticalAlign::Baseline
+        VerticalAlign::Keyword(VerticalAlignKeyword::Baseline)
     }
 }
 
diff --git a/components/style/values/generics/counters.rs b/components/style/values/generics/counters.rs
index 17ac687a670..fbb6927b9f1 100644
--- a/components/style/values/generics/counters.rs
+++ b/components/style/values/generics/counters.rs
@@ -45,7 +45,7 @@ pub struct CounterPair<Integer> {
     ToResolvedValue,
     ToShmem,
 )]
-pub struct CounterIncrement<I>(Counters<I>);
+pub struct CounterIncrement<I>(pub Counters<I>);
 
 impl<I> CounterIncrement<I> {
     /// Returns a new value for `counter-increment`.
@@ -77,7 +77,7 @@ impl<I> Deref for CounterIncrement<I> {
     ToResolvedValue,
     ToShmem,
 )]
-pub struct CounterSetOrReset<I>(Counters<I>);
+pub struct CounterSetOrReset<I>(pub Counters<I>);
 
 impl<I> CounterSetOrReset<I> {
     /// Returns a new value for `counter-set` / `counter-reset`.
@@ -102,6 +102,7 @@ impl<I> Deref for CounterSetOrReset<I> {
 #[derive(
     Clone,
     Debug,
+    Default,
     MallocSizeOf,
     PartialEq,
     SpecifiedValueInfo,
@@ -112,10 +113,13 @@ impl<I> Deref for CounterSetOrReset<I> {
 )]
 pub struct Counters<I>(#[css(iterable, if_empty = "none")] Box<[CounterPair<I>]>);
 
-impl<I> Default for Counters<I> {
+impl<I> Counters<I> {
+    /// Move out the Box into a vector. This could just return the Box<>, but
+    /// Vec<> is a bit more convenient because Box<[T]> doesn't implement
+    /// IntoIter: https://github.com/rust-lang/rust/issues/59878
     #[inline]
-    fn default() -> Self {
-        Counters(vec![].into_boxed_slice())
+    pub fn into_vec(self) -> Vec<CounterPair<I>> {
+        self.0.into_vec()
     }
 }
 
diff --git a/components/style/values/generics/image.rs b/components/style/values/generics/image.rs
index 71d6a1b3849..1442ce604a2 100644
--- a/components/style/values/generics/image.rs
+++ b/components/style/values/generics/image.rs
@@ -135,13 +135,23 @@ pub enum ShapeExtent {
 #[derive(
     Clone, Copy, Debug, MallocSizeOf, PartialEq, ToComputedValue, ToCss, ToResolvedValue, ToShmem,
 )]
-pub enum GradientItem<Color, LengthPercentage> {
-    /// A color stop.
-    ColorStop(ColorStop<Color, LengthPercentage>),
+#[repr(C, u8)]
+pub enum GenericGradientItem<Color, LengthPercentage> {
+    /// A simple color stop, without position.
+    SimpleColorStop(Color),
+    /// A complex color stop, with a position.
+    ComplexColorStop {
+        /// The color for the stop.
+        color: Color,
+        /// The position for the stop.
+        position: LengthPercentage,
+    },
     /// An interpolation hint.
     InterpolationHint(LengthPercentage),
 }
 
+pub use self::GenericGradientItem as GradientItem;
+
 /// A color stop.
 /// <https://drafts.csswg.org/css-images/#typedef-color-stop-list>
 #[derive(
@@ -154,6 +164,20 @@ pub struct ColorStop<Color, LengthPercentage> {
     pub position: Option<LengthPercentage>,
 }
 
+impl<Color, LengthPercentage> ColorStop<Color, LengthPercentage> {
+    /// Convert the color stop into an appropriate `GradientItem`.
+    #[inline]
+    pub fn into_item(self) -> GradientItem<Color, LengthPercentage> {
+        match self.position {
+            Some(position) => GradientItem::ComplexColorStop {
+                color: self.color,
+                position,
+            },
+            None => GradientItem::SimpleColorStop(self.color),
+        }
+    }
+}
+
 /// Specified values for a paint worklet.
 /// <https://drafts.css-houdini.org/css-paint-api/>
 #[cfg_attr(feature = "servo", derive(MallocSizeOf))]
diff --git a/components/style/values/specified/box.rs b/components/style/values/specified/box.rs
index c90c270c59f..31a79c141e1 100644
--- a/components/style/values/specified/box.rs
+++ b/components/style/values/specified/box.rs
@@ -10,7 +10,7 @@ use crate::properties::{LonghandId, PropertyDeclarationId, PropertyFlags};
 use crate::properties::{PropertyId, ShorthandId};
 use crate::values::generics::box_::AnimationIterationCount as GenericAnimationIterationCount;
 use crate::values::generics::box_::Perspective as GenericPerspective;
-use crate::values::generics::box_::VerticalAlign as GenericVerticalAlign;
+use crate::values::generics::box_::{GenericVerticalAlign, VerticalAlignKeyword};
 use crate::values::specified::length::{LengthPercentage, NonNegativeLength};
 use crate::values::specified::{AllowQuirks, Number};
 use crate::values::{CustomIdent, KeyframesName};
@@ -280,20 +280,9 @@ impl Parse for VerticalAlign {
             return Ok(GenericVerticalAlign::Length(lp));
         }
 
-        try_match_ident_ignore_ascii_case! { input,
-            "baseline" => Ok(GenericVerticalAlign::Baseline),
-            "sub" => Ok(GenericVerticalAlign::Sub),
-            "super" => Ok(GenericVerticalAlign::Super),
-            "top" => Ok(GenericVerticalAlign::Top),
-            "text-top" => Ok(GenericVerticalAlign::TextTop),
-            "middle" => Ok(GenericVerticalAlign::Middle),
-            "bottom" => Ok(GenericVerticalAlign::Bottom),
-            "text-bottom" => Ok(GenericVerticalAlign::TextBottom),
-            #[cfg(feature = "gecko")]
-            "-moz-middle-with-baseline" => {
-                Ok(GenericVerticalAlign::MozMiddleWithBaseline)
-            },
-        }
+        Ok(GenericVerticalAlign::Keyword(VerticalAlignKeyword::parse(
+            input,
+        )?))
     }
 }
 
diff --git a/components/style/values/specified/image.rs b/components/style/values/specified/image.rs
index 572db0ca1eb..ad2ecb18086 100644
--- a/components/style/values/specified/image.rs
+++ b/components/style/values/specified/image.rs
@@ -8,6 +8,8 @@
 //! [image]: https://drafts.csswg.org/css-images/#image-values
 
 use crate::custom_properties::SpecifiedValue;
+#[cfg(feature = "gecko")]
+use crate::gecko_bindings::structs;
 use crate::parser::{Parse, ParserContext};
 #[cfg(feature = "gecko")]
 use crate::values::computed::{Context, Position as ComputedPosition, ToComputedValue};
@@ -266,16 +268,6 @@ impl Parse for Gradient {
             },
         };
 
-        #[cfg(feature = "gecko")]
-        {
-            use crate::gecko_bindings::structs;
-            if compat_mode == CompatMode::Moz &&
-                !unsafe { structs::StaticPrefs_sVarCache_layout_css_prefixes_gradients }
-            {
-                return Err(input.new_custom_error(StyleParseErrorKind::UnexpectedFunction(func)));
-            }
-        }
-
         let (kind, items) = input.parse_nested_block(|i| {
             let shape = match shape {
                 Shape::Linear => GradientKind::parse_linear(context, i, &mut compat_mode)?,
@@ -492,24 +484,24 @@ impl Gradient {
                     if reverse_stops {
                         p.reverse();
                     }
-                    Ok(generic::GradientItem::ColorStop(generic::ColorStop {
-                        color: color,
-                        position: Some(p.into()),
-                    }))
+                    Ok(generic::GradientItem::ComplexColorStop {
+                        color,
+                        position: p.into(),
+                    })
                 })
             })
             .unwrap_or(vec![]);
 
         if items.is_empty() {
             items = vec![
-                generic::GradientItem::ColorStop(generic::ColorStop {
+                generic::GradientItem::ComplexColorStop {
                     color: Color::transparent().into(),
-                    position: Some(Percentage::zero().into()),
-                }),
-                generic::GradientItem::ColorStop(generic::ColorStop {
+                    position: Percentage::zero().into(),
+                },
+                generic::GradientItem::ComplexColorStop {
                     color: Color::transparent().into(),
-                    position: Some(Percentage::hundred().into()),
-                }),
+                    position: Percentage::hundred().into(),
+                },
             ];
         } else if items.len() == 1 {
             let first = items[0].clone();
@@ -518,13 +510,16 @@ impl Gradient {
             items.sort_by(|a, b| {
                 match (a, b) {
                     (
-                        &generic::GradientItem::ColorStop(ref a),
-                        &generic::GradientItem::ColorStop(ref b),
-                    ) => match (&a.position, &b.position) {
-                        (
-                            &Some(LengthPercentage::Percentage(a)),
-                            &Some(LengthPercentage::Percentage(b)),
-                        ) => {
+                        &generic::GradientItem::ComplexColorStop {
+                            position: ref a_position,
+                            ..
+                        },
+                        &generic::GradientItem::ComplexColorStop {
+                            position: ref b_position,
+                            ..
+                        },
+                    ) => match (a_position, b_position) {
+                        (&LengthPercentage::Percentage(a), &LengthPercentage::Percentage(b)) => {
                             return a.0.partial_cmp(&b.0).unwrap_or(Ordering::Equal);
                         },
                         _ => {},
@@ -548,6 +543,16 @@ impl Gradient {
     }
 }
 
+#[inline]
+fn simple_moz_gradient() -> bool {
+    #[cfg(feature = "gecko")]
+    unsafe {
+        return structs::StaticPrefs_sVarCache_layout_css_simple_moz_gradient_enabled;
+    }
+    #[cfg(not(feature = "gecko"))]
+    return false;
+}
+
 impl GradientKind {
     /// Parses a linear gradient.
     /// CompatMode can change during `-moz-` prefixed gradient parsing if it come across a `to` keyword.
@@ -583,16 +588,6 @@ impl GradientKind {
                 });
                 (shape, position.ok(), None, None)
             },
-            CompatMode::WebKit => {
-                let position = input.try(|i| Position::parse(context, i));
-                let shape = input.try(|i| {
-                    if position.is_ok() {
-                        i.expect_comma()?;
-                    }
-                    EndingShape::parse(context, i, *compat_mode)
-                });
-                (shape, position.ok(), None, None)
-            },
             // The syntax of `-moz-` prefixed radial gradient is:
             // -moz-radial-gradient(
             //   [ [ <position> || <angle> ]?  [ ellipse | [ <length> | <percentage> ]{2} ] , |
@@ -603,7 +598,7 @@ impl GradientKind {
             // where <extent-keyword> = closest-corner | closest-side | farthest-corner | farthest-side |
             //                          cover | contain
             // and <color-stop>     = <color> [ <percentage> | <length> ]?
-            CompatMode::Moz => {
+            CompatMode::Moz if !simple_moz_gradient() => {
                 let mut position = input.try(|i| LegacyPosition::parse(context, i));
                 let angle = input.try(|i| Angle::parse(context, i)).ok();
                 if position.is_err() {
@@ -619,6 +614,16 @@ impl GradientKind {
 
                 (shape, None, angle, position.ok())
             },
+            _ => {
+                let position = input.try(|i| Position::parse(context, i));
+                let shape = input.try(|i| {
+                    if position.is_ok() {
+                        i.expect_comma()?;
+                    }
+                    EndingShape::parse(context, i, *compat_mode)
+                });
+                (shape, position.ok(), None, None)
+            },
         };
 
         if shape.is_ok() || position.is_some() || angle.is_some() || moz_position.is_some() {
@@ -631,7 +636,7 @@ impl GradientKind {
 
         #[cfg(feature = "gecko")]
         {
-            if *compat_mode == CompatMode::Moz {
+            if *compat_mode == CompatMode::Moz && !simple_moz_gradient() {
                 // If this form can be represented in Modern mode, then convert the compat_mode to Modern.
                 if angle.is_none() {
                     *compat_mode = CompatMode::Modern;
@@ -751,7 +756,7 @@ impl LineDirection {
         input: &mut Parser<'i, 't>,
         compat_mode: &mut CompatMode,
     ) -> Result<Self, ParseError<'i>> {
-        let mut _angle = if *compat_mode == CompatMode::Moz {
+        let mut _angle = if *compat_mode == CompatMode::Moz && !simple_moz_gradient() {
             input.try(|i| Angle::parse(context, i)).ok()
         } else {
             // Gradients allow unitless zero angles as an exception, see:
@@ -784,7 +789,7 @@ impl LineDirection {
             #[cfg(feature = "gecko")]
             {
                 // `-moz-` prefixed linear gradient can be both Angle and Position.
-                if *compat_mode == CompatMode::Moz {
+                if *compat_mode == CompatMode::Moz && !simple_moz_gradient() {
                     let position = i.try(|i| LegacyPosition::parse(context, i)).ok();
                     if _angle.is_none() {
                         _angle = i.try(|i| Angle::parse(context, i)).ok();
@@ -874,7 +879,7 @@ impl EndingShape {
         }
         // -moz- prefixed radial gradient doesn't allow EndingShape's Length or LengthPercentage
         // to come before shape keyword. Otherwise it conflicts with <position>.
-        if compat_mode != CompatMode::Moz {
+        if compat_mode != CompatMode::Moz || simple_moz_gradient() {
             if let Ok(length) = input.try(|i| Length::parse(context, i)) {
                 if let Ok(y) = input.try(|i| LengthPercentage::parse(context, i)) {
                     if compat_mode == CompatMode::Modern {
@@ -958,13 +963,16 @@ impl GradientItem {
 
                 if let Ok(multi_position) = input.try(|i| LengthPercentage::parse(context, i)) {
                     let stop_color = stop.color.clone();
-                    items.push(generic::GradientItem::ColorStop(stop));
-                    items.push(generic::GradientItem::ColorStop(ColorStop {
-                        color: stop_color,
-                        position: Some(multi_position),
-                    }));
+                    items.push(stop.into_item());
+                    items.push(
+                        ColorStop {
+                            color: stop_color,
+                            position: Some(multi_position),
+                        }
+                        .into_item(),
+                    );
                 } else {
-                    items.push(generic::GradientItem::ColorStop(stop));
+                    items.push(stop.into_item());
                 }
 
                 seen_stop = true;
diff --git a/components/style/values/specified/mod.rs b/components/style/values/specified/mod.rs
index 64a76ccc582..7dbb93f6319 100644
--- a/components/style/values/specified/mod.rs
+++ b/components/style/values/specified/mod.rs
@@ -80,6 +80,7 @@ pub use self::svg::{SVGLength, SVGOpacity, SVGPaint, SVGPaintKind};
 pub use self::svg::{SVGPaintOrder, SVGStrokeDashArray, SVGWidth};
 pub use self::svg_path::SVGPathData;
 pub use self::table::XSpan;
+pub use self::text::TextTransform;
 pub use self::text::{InitialLetter, LetterSpacing, LineHeight, TextAlign};
 pub use self::text::{OverflowWrap, TextEmphasisPosition, TextEmphasisStyle, WordBreak};
 pub use self::text::{TextAlignKeyword, TextDecorationLine, TextOverflow, WordSpacing};
diff --git a/components/style/values/specified/position.rs b/components/style/values/specified/position.rs
index f75af3f5585..c0ac56c9822 100644
--- a/components/style/values/specified/position.rs
+++ b/components/style/values/specified/position.rs
@@ -460,6 +460,11 @@ pub enum AutoFlow {
     Column,
 }
 
+/// If `dense` is specified, `row` is implied.
+fn is_row_dense(autoflow: &AutoFlow, dense: &bool) -> bool {
+    *autoflow == AutoFlow::Row && *dense
+}
+
 #[derive(
     Clone,
     Copy,
@@ -477,6 +482,7 @@ pub enum AutoFlow {
 /// specifying exactly how auto-placed items get flowed into the grid
 pub struct GridAutoFlow {
     /// Specifiy how auto-placement algorithm fills each `row` or `column` in turn
+    #[css(contextual_skip_if = "is_row_dense")]
     pub autoflow: AutoFlow,
     /// Specify use `dense` packing algorithm or not
     #[css(represents_keyword)]
diff --git a/components/style/values/specified/text.rs b/components/style/values/specified/text.rs
index 9d4c6ae649c..6be20f58618 100644
--- a/components/style/values/specified/text.rs
+++ b/components/style/values/specified/text.rs
@@ -351,6 +351,177 @@ impl TextDecorationLine {
     }
 }
 
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C)]
+/// Specified value of the text-transform property, stored in two parts:
+/// the case-related transforms (mutually exclusive, only one may be in effect), and others (non-exclusive).
+pub struct TextTransform {
+    /// Case transform, if any.
+    pub case_: TextTransformCase,
+    /// Non-case transforms.
+    pub other_: TextTransformOther,
+}
+
+impl TextTransform {
+    #[inline]
+    /// Returns the initial value of text-transform
+    pub fn none() -> Self {
+        TextTransform {
+            case_: TextTransformCase::None,
+            other_: TextTransformOther::empty(),
+        }
+    }
+    #[inline]
+    /// Returns whether the value is 'none'
+    pub fn is_none(&self) -> bool {
+        self.case_ == TextTransformCase::None && self.other_.is_empty()
+    }
+}
+
+impl Parse for TextTransform {
+    fn parse<'i, 't>(
+        _context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        let mut result = TextTransform::none();
+
+        // Case keywords are mutually exclusive; other transforms may co-occur.
+        loop {
+            let location = input.current_source_location();
+            let ident = match input.next() {
+                Ok(&Token::Ident(ref ident)) => ident,
+                Ok(other) => return Err(location.new_unexpected_token_error(other.clone())),
+                Err(..) => break,
+            };
+
+            match_ignore_ascii_case! { ident,
+                "none" if result.is_none() => {
+                    return Ok(result);
+                },
+                "uppercase" if result.case_ == TextTransformCase::None => {
+                    result.case_ = TextTransformCase::Uppercase
+                },
+                "lowercase" if result.case_ == TextTransformCase::None => {
+                    result.case_ = TextTransformCase::Lowercase
+                },
+                "capitalize" if result.case_ == TextTransformCase::None => {
+                    result.case_ = TextTransformCase::Capitalize
+                },
+                "full-width" if !result.other_.intersects(TextTransformOther::FULL_WIDTH) => {
+                    result.other_.insert(TextTransformOther::FULL_WIDTH)
+                },
+                "full-size-kana" if !result.other_.intersects(TextTransformOther::FULL_SIZE_KANA) => {
+                    result.other_.insert(TextTransformOther::FULL_SIZE_KANA)
+                }
+                _ => return Err(location.new_custom_error(
+                    SelectorParseErrorKind::UnexpectedIdent(ident.clone())
+                )),
+            }
+        }
+
+        if result.is_none() {
+            Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError))
+        } else {
+            Ok(result)
+        }
+    }
+}
+
+impl ToCss for TextTransform {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        if self.is_none() {
+            return dest.write_str("none");
+        }
+
+        if self.case_ != TextTransformCase::None {
+            self.case_.to_css(dest)?;
+            if !self.other_.is_empty() {
+                dest.write_str(" ")?;
+            }
+        }
+
+        self.other_.to_css(dest)
+    }
+}
+
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C)]
+/// Specified keyword values for case transforms in the text-transform property. (These are exclusive.)
+pub enum TextTransformCase {
+    /// No case transform.
+    None,
+    /// All uppercase.
+    Uppercase,
+    /// All lowercase.
+    Lowercase,
+    /// Capitalize each word.
+    Capitalize,
+}
+
+bitflags! {
+    #[derive(MallocSizeOf, SpecifiedValueInfo, ToComputedValue, ToResolvedValue, ToShmem)]
+    #[value_info(other_values = "none,full-width,full-size-kana")]
+    #[repr(C)]
+    /// Specified keyword values for non-case transforms in the text-transform property. (Non-exclusive.)
+    pub struct TextTransformOther: u8 {
+        /// full-width
+        const FULL_WIDTH = 1 << 0;
+        /// full-size-kana
+        const FULL_SIZE_KANA = 1 << 1;
+    }
+}
+
+impl ToCss for TextTransformOther {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        let mut writer = SequenceWriter::new(dest, " ");
+        let mut any = false;
+        macro_rules! maybe_write {
+            ($ident:ident => $str:expr) => {
+                if self.contains(TextTransformOther::$ident) {
+                    writer.raw_item($str)?;
+                    any = true;
+                }
+            };
+        }
+
+        maybe_write!(FULL_WIDTH => "full-width");
+        maybe_write!(FULL_SIZE_KANA => "full-size-kana");
+
+        debug_assert!(any || self.is_empty());
+
+        Ok(())
+    }
+}
+
 /// Specified value of text-align keyword value.
 #[derive(
     Clone,
@@ -394,8 +565,9 @@ pub enum TextAlignKeyword {
 }
 
 /// Specified value of text-align property.
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[derive(Clone, Copy, Debug, Eq, Hash, Parse, PartialEq, SpecifiedValueInfo, ToCss, ToShmem)]
+#[derive(
+    Clone, Copy, Debug, Eq, Hash, MallocSizeOf, Parse, PartialEq, SpecifiedValueInfo, ToCss, ToShmem,
+)]
 pub enum TextAlign {
     /// Keyword value of text-align property.
     Keyword(TextAlignKeyword),
diff --git a/tests/wpt/metadata/css/css-text/parsing/text-transform-valid.html.ini b/tests/wpt/metadata/css/css-text/parsing/text-transform-valid.html.ini
deleted file mode 100644
index c9e0f5c67db..00000000000
--- a/tests/wpt/metadata/css/css-text/parsing/text-transform-valid.html.ini
+++ /dev/null
@@ -1,43 +0,0 @@
-[text-transform-valid.html]
-  [e.style['text-transform'\] = "full-size-kana full-width capitalize" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-width" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "capitalize full-width" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-size-kana full-width" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "capitalize full-width full-size-kana" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-width full-size-kana" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-size-kana lowercase full-width" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-size-kana capitalize" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "lowercase full-size-kana full-width" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-width lowercase" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-size-kana" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-width full-size-kana uppercase" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "uppercase full-size-kana" should set the property value]
-    expected: FAIL
-
-  [e.style['text-transform'\] = "full-width uppercase full-size-kana" should set the property value]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/css/css-variables/wide-keyword-fallback.html.ini b/tests/wpt/metadata/css/css-variables/wide-keyword-fallback.html.ini
deleted file mode 100644
index 3bdd09230d8..00000000000
--- a/tests/wpt/metadata/css/css-variables/wide-keyword-fallback.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[wide-keyword-fallback.html]
-  expected: FAIL