style: Implement CSS parsing for the @font-palette-values rule

Not yet hooked up to any rendering functionality.

The intention is for both the @font-palette-values at-rule and the font-palette property
to be behind the same pref being introduced here.

Differential Revision: https://phabricator.services.mozilla.com/D157953

12 files changed, 353 insertions, 30 deletions

diff --git a/components/style/error_reporting.rs b/components/style/error_reporting.rs
index 752f68b5bdc..042838f3394 100644
--- a/components/style/error_reporting.rs
+++ b/components/style/error_reporting.rs
@@ -26,6 +26,8 @@ pub enum ContextualParseError<'a> {
     UnsupportedFontFaceDescriptor(&'a str, ParseError<'a>),
     /// A font feature values descriptor was not recognized.
     UnsupportedFontFeatureValuesDescriptor(&'a str, ParseError<'a>),
+    /// A font palette values descriptor was not recognized.
+    UnsupportedFontPaletteValuesDescriptor(&'a str, ParseError<'a>),
     /// A keyframe rule was not valid.
     InvalidKeyframeRule(&'a str, ParseError<'a>),
     /// A font feature values rule was not valid.
@@ -149,6 +151,14 @@ impl<'a> fmt::Display for ContextualParseError<'a> {
                 )?;
                 parse_error_to_str(err, f)
             },
+            ContextualParseError::UnsupportedFontPaletteValuesDescriptor(decl, ref err) => {
+                write!(
+                    f,
+                    "Unsupported @font-palette-values descriptor declaration: '{}', ",
+                    decl
+                )?;
+                parse_error_to_str(err, f)
+            },
             ContextualParseError::InvalidKeyframeRule(rule, ref err) => {
                 write!(f, "Invalid keyframe rule: '{}', ", rule)?;
                 parse_error_to_str(err, f)
diff --git a/components/style/gecko/arc_types.rs b/components/style/gecko/arc_types.rs
index bb1d88d8a55..fc712422536 100644
--- a/components/style/gecko/arc_types.rs
+++ b/components/style/gecko/arc_types.rs
@@ -12,10 +12,10 @@ use crate::gecko::url::CssUrlData;
 use crate::gecko_bindings::structs::{
     RawServoAnimationValue, RawServoContainerRule, RawServoCounterStyleRule, RawServoCssUrlData,
     RawServoDeclarationBlock, RawServoFontFaceRule, RawServoFontFeatureValuesRule,
-    RawServoImportRule, RawServoKeyframe, RawServoKeyframesRule, RawServoLayerBlockRule,
-    RawServoLayerStatementRule, RawServoMediaList, RawServoMediaRule, RawServoMozDocumentRule,
-    RawServoNamespaceRule, RawServoPageRule, RawServoStyleRule, RawServoStyleSheetContents,
-    RawServoSupportsRule, ServoCssRules,
+    RawServoFontPaletteValuesRule, RawServoImportRule, RawServoKeyframe, RawServoKeyframesRule,
+    RawServoLayerBlockRule, RawServoLayerStatementRule, RawServoMediaList, RawServoMediaRule,
+    RawServoMozDocumentRule, RawServoNamespaceRule, RawServoPageRule, RawServoStyleRule,
+    RawServoStyleSheetContents, RawServoSupportsRule, ServoCssRules,
 };
 use crate::gecko_bindings::sugar::ownership::{HasArcFFI, HasFFI, Strong};
 use crate::media_queries::MediaList;
@@ -25,8 +25,8 @@ use crate::shared_lock::Locked;
 use crate::stylesheets::keyframes_rule::Keyframe;
 use crate::stylesheets::{
     ContainerRule, CounterStyleRule, CssRules, DocumentRule, FontFaceRule, FontFeatureValuesRule,
-    ImportRule, KeyframesRule, LayerBlockRule, LayerStatementRule, MediaRule, NamespaceRule,
-    PageRule, StyleRule, StylesheetContents, SupportsRule,
+    FontPaletteValuesRule, ImportRule, KeyframesRule, LayerBlockRule, LayerStatementRule,
+    MediaRule, NamespaceRule, PageRule, StyleRule, StylesheetContents, SupportsRule,
 };
 use servo_arc::{Arc, ArcBorrow};
 use std::{mem, ptr};
@@ -104,6 +104,9 @@ impl_arc_ffi!(Locked<DocumentRule> => RawServoMozDocumentRule
 impl_arc_ffi!(Locked<FontFeatureValuesRule> => RawServoFontFeatureValuesRule
               [Servo_FontFeatureValuesRule_AddRef, Servo_FontFeatureValuesRule_Release]);
 
+impl_arc_ffi!(Locked<FontPaletteValuesRule> => RawServoFontPaletteValuesRule
+              [Servo_FontPaletteValuesRule_AddRef, Servo_FontPaletteValuesRule_Release]);
+
 impl_arc_ffi!(Locked<FontFaceRule> => RawServoFontFaceRule
               [Servo_FontFaceRule_AddRef, Servo_FontFaceRule_Release]);
 
diff --git a/components/style/invalidation/stylesheets.rs b/components/style/invalidation/stylesheets.rs
index b367e18daa9..f18fe7942c5 100644
--- a/components/style/invalidation/stylesheets.rs
+++ b/components/style/invalidation/stylesheets.rs
@@ -552,6 +552,7 @@ impl StylesheetInvalidationSet {
             Page(..) |
             Viewport(..) |
             FontFeatureValues(..) |
+            FontPaletteValues(..) |
             LayerStatement(..) |
             FontFace(..) |
             Keyframes(..) |
@@ -632,7 +633,8 @@ impl StylesheetInvalidationSet {
                     // existing elements.
                 }
             },
-            CounterStyle(..) | Page(..) | Viewport(..) | FontFeatureValues(..) => {
+            CounterStyle(..) | Page(..) | Viewport(..) | FontFeatureValues(..) |
+            FontPaletteValues(..) => {
                 debug!(
                     " > Found unsupported rule, marking the whole subtree \
                      invalid."
diff --git a/components/style/stylesheets/font_feature_values_rule.rs b/components/style/stylesheets/font_feature_values_rule.rs
index dc128e77fb0..784628f0dde 100644
--- a/components/style/stylesheets/font_feature_values_rule.rs
+++ b/components/style/stylesheets/font_feature_values_rule.rs
@@ -275,23 +275,6 @@ macro_rules! font_feature_values_blocks {
                 rule
             }
 
-            /// Prints font family names.
-            pub fn font_family_to_css<W>(
-                &self,
-                dest: &mut CssWriter<W>,
-            ) -> fmt::Result
-            where
-                W: Write,
-            {
-                let mut iter = self.family_names.iter();
-                iter.next().unwrap().to_css(dest)?;
-                for val in iter {
-                    dest.write_str(", ")?;
-                    val.to_css(dest)?;
-                }
-                Ok(())
-            }
-
             /// Prints inside of `@font-feature-values` block.
             pub fn value_to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
             where
@@ -349,7 +332,7 @@ macro_rules! font_feature_values_blocks {
         impl ToCssWithGuard for FontFeatureValuesRule {
             fn to_css(&self, _guard: &SharedRwLockReadGuard, dest: &mut CssStringWriter) -> fmt::Result {
                 dest.write_str("@font-feature-values ")?;
-                self.font_family_to_css(&mut CssWriter::new(dest))?;
+                self.family_names.to_css(&mut CssWriter::new(dest))?;
                 dest.write_str(" {\n")?;
                 self.value_to_css(&mut CssWriter::new(dest))?;
                 dest.write_str("}")
diff --git a/components/style/stylesheets/font_palette_values_rule.rs b/components/style/stylesheets/font_palette_values_rule.rs
new file mode 100644
index 00000000000..95d62443321
--- /dev/null
+++ b/components/style/stylesheets/font_palette_values_rule.rs
@@ -0,0 +1,215 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! The [`@font-palette-values`][font-palette-values] at-rule.
+//!
+//! [font-palette-values]: https://drafts.csswg.org/css-fonts/#font-palette-values
+
+use crate::error_reporting::ContextualParseError;
+use crate::parser::{Parse, ParserContext};
+use crate::shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
+use crate::str::CssStringWriter;
+use crate::values::computed::font::FamilyName;
+use crate::values::specified::Color as SpecifiedColor;
+use crate::values::specified::NonNegativeInteger;
+use crate::values::DashedIdent;
+use cssparser::{AtRuleParser, CowRcStr};
+use cssparser::{DeclarationParser, DeclarationListParser, Parser};
+use cssparser::{QualifiedRuleParser, SourceLocation};
+use std::fmt::{self, Write};
+use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss};
+use style_traits::{Comma, OneOrMoreSeparated};
+use selectors::parser::SelectorParseErrorKind;
+use crate::stylesheets::font_feature_values_rule::parse_family_name_list;
+
+#[allow(missing_docs)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToShmem)]
+pub struct FontPaletteOverrideColor {
+    index: NonNegativeInteger,
+    color: SpecifiedColor,
+}
+
+impl Parse for FontPaletteOverrideColor {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<FontPaletteOverrideColor, ParseError<'i>> {
+        let index = NonNegativeInteger::parse(context, input)?;
+        let location = input.current_source_location();
+        let color = SpecifiedColor::parse(context, input)?;
+        // Only absolute colors are accepted here.
+        if let SpecifiedColor::Numeric { parsed: _, authored: _ } = color {
+            Ok(FontPaletteOverrideColor{ index, color })
+        } else {
+            Err(location.new_custom_error(StyleParseErrorKind::UnspecifiedError))
+        }
+    }
+}
+
+impl ToCss for FontPaletteOverrideColor {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        self.index.to_css(dest)?;
+        dest.write_str(" ")?;
+        self.color.to_css(dest)
+    }
+}
+
+impl OneOrMoreSeparated for FontPaletteOverrideColor {
+    type S = Comma;
+}
+
+impl OneOrMoreSeparated for FamilyName {
+    type S = Comma;
+}
+
+#[allow(missing_docs)]
+#[derive(Clone, Debug, MallocSizeOf, Parse, PartialEq, ToCss, ToShmem)]
+pub enum FontPaletteBase {
+    Light,
+    Dark,
+    Index(NonNegativeInteger),
+}
+
+/// The [`@font-palette-values`][font-palette-values] at-rule.
+///
+/// [font-palette-values]: https://drafts.csswg.org/css-fonts/#font-palette-values
+#[derive(Clone, Debug, PartialEq, ToShmem)]
+pub struct FontPaletteValuesRule {
+    /// Palette name.
+    pub name: DashedIdent,
+    /// Font family list for @font-palette-values rule.
+    /// Family names cannot contain generic families. FamilyName
+    /// also accepts only non-generic names.
+    pub family_names: Vec<FamilyName>,
+    /// The base palette.
+    pub base_palette: Option<FontPaletteBase>,
+    /// The list of override colors.
+    pub override_colors: Vec<FontPaletteOverrideColor>,
+    /// The line and column of the rule's source code.
+    pub source_location: SourceLocation,
+}
+
+impl FontPaletteValuesRule {
+    /// Creates an empty FontPaletteValuesRule with given location and name.
+    fn new(name: DashedIdent, location: SourceLocation) -> Self {
+        FontPaletteValuesRule {
+            name,
+            family_names: vec![],
+            base_palette: None,
+            override_colors: vec![],
+            source_location: location,
+        }
+    }
+
+    /// Parses a `FontPaletteValuesRule`.
+    pub fn parse(
+        context: &ParserContext,
+        input: &mut Parser,
+        name: DashedIdent,
+        location: SourceLocation,
+    ) -> Self {
+        let mut rule = FontPaletteValuesRule::new(name, location);
+        {
+            let parser = FontPaletteValuesDeclarationParser {
+                context: context,
+                rule: &mut rule,
+            };
+            let mut iter = DeclarationListParser::new(input, parser);
+            while let Some(declaration) = iter.next() {
+                if let Err((error, slice)) = declaration {
+                    let location = error.location;
+                    let error = ContextualParseError::UnsupportedFontPaletteValuesDescriptor(slice, error);
+                    context.log_css_error(location, error);
+                }
+            }
+        }
+        rule
+    }
+
+    /// Prints inside of `@font-palette-values` block.
+    fn value_to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        if !self.family_names.is_empty() {
+            dest.write_str("font-family: ")?;
+            self.family_names.to_css(dest)?;
+            dest.write_str("; ")?;
+        }
+        if let Some(base) = &self.base_palette {
+            dest.write_str("base-palette: ")?;
+            base.to_css(dest)?;
+            dest.write_str("; ")?;
+        }
+        if !self.override_colors.is_empty() {
+            dest.write_str("override-colors: ")?;
+            self.override_colors.to_css(dest)?;
+            dest.write_str("; ")?;
+        }
+        Ok(())
+    }
+}
+
+impl ToCssWithGuard for FontPaletteValuesRule {
+    fn to_css(&self, _guard: &SharedRwLockReadGuard, dest: &mut CssStringWriter) -> fmt::Result {
+        dest.write_str("@font-palette-values ")?;
+        self.name.to_css(&mut CssWriter::new(dest))?;
+        dest.write_str(" { ")?;
+        self.value_to_css(&mut CssWriter::new(dest))?;
+        dest.write_str("}")
+    }
+}
+
+/// Parser for declarations in `FontPaletteValuesRule`.
+struct FontPaletteValuesDeclarationParser<'a> {
+    context: &'a ParserContext<'a>,
+    rule: &'a mut FontPaletteValuesRule,
+}
+
+impl<'a, 'i> AtRuleParser<'i> for FontPaletteValuesDeclarationParser<'a> {
+    type Prelude = ();
+    type AtRule = ();
+    type Error = StyleParseErrorKind<'i>;
+}
+
+impl<'a, 'i> QualifiedRuleParser<'i> for FontPaletteValuesDeclarationParser<'a> {
+    type Prelude = ();
+    type QualifiedRule = ();
+    type Error = StyleParseErrorKind<'i>;
+}
+
+fn parse_override_colors<'i, 't>(
+    context: &ParserContext,
+    input: &mut Parser<'i, 't>,
+) -> Result<Vec<FontPaletteOverrideColor>, ParseError<'i>> {
+    input.parse_comma_separated(|i| FontPaletteOverrideColor::parse(context, i))
+}
+
+impl<'a, 'b, 'i> DeclarationParser<'i> for FontPaletteValuesDeclarationParser<'a> {
+   type Declaration = ();
+   type Error = StyleParseErrorKind<'i>;
+
+   fn parse_value<'t>(
+       &mut self,
+       name: CowRcStr<'i>,
+       input: &mut Parser<'i, 't>,
+    ) -> Result<(), ParseError<'i>> {
+        match_ignore_ascii_case! { &*name,
+            "font-family" => {
+                self.rule.family_names = parse_family_name_list(self.context, input)?
+            },
+            "base-palette" => {
+                self.rule.base_palette = Some(input.parse_entirely(|i| FontPaletteBase::parse(self.context, i))?)
+            },
+            "override-colors" => {
+                self.rule.override_colors = parse_override_colors(self.context, input)?
+            },
+            _ => return Err(input.new_custom_error(SelectorParseErrorKind::UnexpectedIdent(name.clone()))),
+        }
+        Ok(())
+    }
+}
diff --git a/components/style/stylesheets/mod.rs b/components/style/stylesheets/mod.rs
index 9625ad62cb3..01b0c07d7a9 100644
--- a/components/style/stylesheets/mod.rs
+++ b/components/style/stylesheets/mod.rs
@@ -10,6 +10,7 @@ mod counter_style_rule;
 mod document_rule;
 mod font_face_rule;
 pub mod font_feature_values_rule;
+pub mod font_palette_values_rule;
 pub mod import_rule;
 pub mod keyframes_rule;
 pub mod layer_rule;
@@ -50,6 +51,7 @@ pub use self::counter_style_rule::CounterStyleRule;
 pub use self::document_rule::DocumentRule;
 pub use self::font_face_rule::FontFaceRule;
 pub use self::font_feature_values_rule::FontFeatureValuesRule;
+pub use self::font_palette_values_rule::FontPaletteValuesRule;
 pub use self::import_rule::ImportRule;
 pub use self::keyframes_rule::KeyframesRule;
 pub use self::layer_rule::{LayerBlockRule, LayerStatementRule};
@@ -256,6 +258,7 @@ pub enum CssRule {
     Container(Arc<Locked<ContainerRule>>),
     FontFace(Arc<Locked<FontFaceRule>>),
     FontFeatureValues(Arc<Locked<FontFeatureValuesRule>>),
+    FontPaletteValues(Arc<Locked<FontPaletteValuesRule>>),
     CounterStyle(Arc<Locked<CounterStyleRule>>),
     Viewport(Arc<Locked<ViewportRule>>),
     Keyframes(Arc<Locked<KeyframesRule>>),
@@ -293,6 +296,7 @@ impl CssRule {
 
             CssRule::FontFace(_) => 0,
             CssRule::FontFeatureValues(_) => 0,
+            CssRule::FontPaletteValues(_) => 0,
             CssRule::CounterStyle(_) => 0,
             CssRule::Viewport(_) => 0,
             CssRule::Keyframes(_) => 0,
@@ -348,6 +352,7 @@ pub enum CssRuleType {
     LayerBlock = 16,
     LayerStatement = 17,
     Container = 18,
+    FontPaletteValues = 19,
 }
 
 #[allow(missing_docs)]
@@ -367,6 +372,7 @@ impl CssRule {
             CssRule::Media(_) => CssRuleType::Media,
             CssRule::FontFace(_) => CssRuleType::FontFace,
             CssRule::FontFeatureValues(_) => CssRuleType::FontFeatureValues,
+            CssRule::FontPaletteValues(_) => CssRuleType::FontPaletteValues,
             CssRule::CounterStyle(_) => CssRuleType::CounterStyle,
             CssRule::Keyframes(_) => CssRuleType::Keyframes,
             CssRule::Namespace(_) => CssRuleType::Namespace,
@@ -474,6 +480,10 @@ impl DeepCloneWithLock for CssRule {
                 let rule = arc.read_with(guard);
                 CssRule::FontFeatureValues(Arc::new(lock.wrap(rule.clone())))
             },
+            CssRule::FontPaletteValues(ref arc) => {
+                let rule = arc.read_with(guard);
+                CssRule::FontPaletteValues(Arc::new(lock.wrap(rule.clone())))
+            },
             CssRule::CounterStyle(ref arc) => {
                 let rule = arc.read_with(guard);
                 CssRule::CounterStyle(Arc::new(lock.wrap(rule.clone())))
@@ -531,6 +541,7 @@ impl ToCssWithGuard for CssRule {
             CssRule::Style(ref lock) => lock.read_with(guard).to_css(guard, dest),
             CssRule::FontFace(ref lock) => lock.read_with(guard).to_css(guard, dest),
             CssRule::FontFeatureValues(ref lock) => lock.read_with(guard).to_css(guard, dest),
+            CssRule::FontPaletteValues(ref lock) => lock.read_with(guard).to_css(guard, dest),
             CssRule::CounterStyle(ref lock) => lock.read_with(guard).to_css(guard, dest),
             CssRule::Viewport(ref lock) => lock.read_with(guard).to_css(guard, dest),
             CssRule::Keyframes(ref lock) => lock.read_with(guard).to_css(guard, dest),
diff --git a/components/style/stylesheets/rule_parser.rs b/components/style/stylesheets/rule_parser.rs
index 659b704161e..9fe2d0c87e9 100644
--- a/components/style/stylesheets/rule_parser.rs
+++ b/components/style/stylesheets/rule_parser.rs
@@ -23,11 +23,12 @@ use crate::stylesheets::stylesheet::Namespaces;
 use crate::stylesheets::supports_rule::SupportsCondition;
 use crate::stylesheets::{
     viewport_rule, AllowImportRules, CorsMode, CssRule, CssRuleType, CssRules, DocumentRule,
-    FontFeatureValuesRule, KeyframesRule, MediaRule, NamespaceRule, PageRule, PageSelectors,
-    RulesMutateError, StyleRule, StylesheetLoader, SupportsRule, ViewportRule,
+    FontFeatureValuesRule, FontPaletteValuesRule, KeyframesRule, MediaRule, NamespaceRule,
+    PageRule, PageSelectors, RulesMutateError, StyleRule, StylesheetLoader, SupportsRule,
+    ViewportRule,
 };
 use crate::values::computed::font::FamilyName;
-use crate::values::{CssUrl, CustomIdent, KeyframesName};
+use crate::values::{CssUrl, CustomIdent, DashedIdent, KeyframesName};
 use crate::{Namespace, Prefix};
 use cssparser::{
     AtRuleParser, BasicParseError, BasicParseErrorKind, CowRcStr, Parser, ParserState,
@@ -184,6 +185,8 @@ pub enum AtRulePrelude {
     FontFace,
     /// A @font-feature-values rule prelude, with its FamilyName list.
     FontFeatureValues(Vec<FamilyName>),
+    /// A @font-palette-values rule prelude, with its identifier.
+    FontPaletteValues(DashedIdent),
     /// A @counter-style rule prelude, with its counter style name.
     CounterStyle(CustomIdent),
     /// A @media rule prelude, with its media queries.
@@ -500,6 +503,10 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                 let family_names = parse_family_name_list(self.context, input)?;
                 AtRulePrelude::FontFeatureValues(family_names)
             },
+            "font-palette-values" if static_prefs::pref!("layout.css.font-palette.enabled") => {
+                let name = DashedIdent::parse(self.context, input)?;
+                AtRulePrelude::FontPaletteValues(name)
+            },
             "counter-style" if cfg!(feature = "gecko") => {
                 let name = parse_counter_style_name_definition(input)?;
                 AtRulePrelude::CounterStyle(name)
@@ -573,6 +580,22 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                     ),
                 ))))
             },
+            AtRulePrelude::FontPaletteValues(name) => {
+                let context = ParserContext::new_with_rule_type(
+                    self.context,
+                    CssRuleType::FontPaletteValues,
+                    self.namespaces,
+                );
+
+                Ok(CssRule::FontPaletteValues(Arc::new(self.shared_lock.wrap(
+                    FontPaletteValuesRule::parse(
+                        &context,
+                        input,
+                        name,
+                        start.source_location(),
+                    ),
+                ))))
+            },
             AtRulePrelude::CounterStyle(name) => {
                 let context = ParserContext::new_with_rule_type(
                     self.context,
diff --git a/components/style/stylesheets/rules_iterator.rs b/components/style/stylesheets/rules_iterator.rs
index 5c90e506714..00c095bb8b8 100644
--- a/components/style/stylesheets/rules_iterator.rs
+++ b/components/style/stylesheets/rules_iterator.rs
@@ -70,7 +70,8 @@ where
             CssRule::Keyframes(_) |
             CssRule::Page(_) |
             CssRule::LayerStatement(_) |
-            CssRule::FontFeatureValues(_) => None,
+            CssRule::FontFeatureValues(_) |
+            CssRule::FontPaletteValues(_) => None,
             CssRule::Import(ref import_rule) => {
                 let import_rule = import_rule.read_with(guard);
                 if !C::process_import(guard, device, quirks_mode, import_rule) {
diff --git a/components/style/stylesheets/stylesheet.rs b/components/style/stylesheets/stylesheet.rs
index 51a1a29ac07..a1a678ec436 100644
--- a/components/style/stylesheets/stylesheet.rs
+++ b/components/style/stylesheets/stylesheet.rs
@@ -381,6 +381,7 @@ impl SanitizationKind {
             CssRule::Keyframes(..) |
             CssRule::Page(..) |
             CssRule::FontFeatureValues(..) |
+            CssRule::FontPaletteValues(..) |
             CssRule::Viewport(..) |
             CssRule::CounterStyle(..) => !is_standard,
         }
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 487f3ade5de..8dfdc91e58b 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -32,7 +32,9 @@ use crate::stylesheets::keyframes_rule::KeyframesAnimation;
 use crate::stylesheets::layer_rule::{LayerName, LayerOrder};
 use crate::stylesheets::viewport_rule::{self, MaybeNew, ViewportRule};
 #[cfg(feature = "gecko")]
-use crate::stylesheets::{CounterStyleRule, FontFaceRule, FontFeatureValuesRule, PageRule};
+use crate::stylesheets::{
+    CounterStyleRule, FontFaceRule, FontFeatureValuesRule, FontPaletteValuesRule, PageRule,
+};
 use crate::stylesheets::{
     CssRule, EffectiveRulesIterator, Origin, OriginSet, PageRule, PerOrigin, PerOriginIter,
 };
@@ -1679,6 +1681,10 @@ pub struct ExtraStyleData {
     #[cfg(feature = "gecko")]
     pub font_feature_values: LayerOrderedVec<Arc<Locked<FontFeatureValuesRule>>>,
 
+    /// A list of effective font-palette-values rules.
+    #[cfg(feature = "gecko")]
+    pub font_palette_values: LayerOrderedVec<Arc<Locked<FontPaletteValuesRule>>>,
+
     /// A map of effective counter-style rules.
     #[cfg(feature = "gecko")]
     pub counter_styles: LayerOrderedMap<Arc<Locked<CounterStyleRule>>>,
@@ -1704,6 +1710,15 @@ impl ExtraStyleData {
         self.font_feature_values.push(rule.clone(), layer);
     }
 
+    /// Add the given @font-palette-values rule.
+    fn add_font_palette_values(
+        &mut self,
+        rule: &Arc<Locked<FontPaletteValuesRule>>,
+        layer: LayerId,
+    ) {
+        self.font_palette_values.push(rule.clone(), layer);
+    }
+
     /// Add the given @counter-style rule.
     fn add_counter_style(
         &mut self,
@@ -1745,6 +1760,7 @@ impl ExtraStyleData {
     fn sort_by_layer(&mut self, layers: &[CascadeLayer]) {
         self.font_faces.sort(layers);
         self.font_feature_values.sort(layers);
+        self.font_palette_values.sort(layers);
         self.counter_styles.sort(layers);
         self.pages.global.sort(layers);
     }
@@ -1754,6 +1770,7 @@ impl ExtraStyleData {
         {
             self.font_faces.clear();
             self.font_feature_values.clear();
+            self.font_palette_values.clear();
             self.counter_styles.clear();
             self.pages.clear();
         }
@@ -1790,6 +1807,7 @@ impl MallocSizeOf for ExtraStyleData {
         let mut n = 0;
         n += self.font_faces.shallow_size_of(ops);
         n += self.font_feature_values.shallow_size_of(ops);
+        n += self.font_palette_values.shallow_size_of(ops);
         n += self.counter_styles.shallow_size_of(ops);
         n += self.pages.shallow_size_of(ops);
         n
@@ -2638,6 +2656,11 @@ impl CascadeData {
                         .add_font_feature_values(rule, containing_rule_state.layer_id);
                 },
                 #[cfg(feature = "gecko")]
+                CssRule::FontPaletteValues(ref rule) => {
+                    self.extra_data
+                        .add_font_palette_values(rule, containing_rule_state.layer_id);
+                },
+                #[cfg(feature = "gecko")]
                 CssRule::CounterStyle(ref rule) => {
                     self.extra_data.add_counter_style(
                         guard,
@@ -2888,6 +2911,7 @@ impl CascadeData {
                 CssRule::Document(..) |
                 CssRule::LayerBlock(..) |
                 CssRule::LayerStatement(..) |
+                CssRule::FontPaletteValues(..) |
                 CssRule::FontFeatureValues(..) => {
                     // Not affected by device changes.
                     continue;
diff --git a/components/style/values/mod.rs b/components/style/values/mod.rs
index 2a0481ab684..c0ddeddd7ee 100644
--- a/components/style/values/mod.rs
+++ b/components/style/values/mod.rs
@@ -492,6 +492,44 @@ impl ToCss for CustomIdent {
     }
 }
 
+/// <https://www.w3.org/TR/css-values-4/#dashed-idents>
+/// This is simply an Atom, but will only parse if the identifier starts with "--".
+#[repr(transparent)]
+#[derive(
+    Clone,
+    Debug,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToResolvedValue,
+    ToShmem,
+)]
+pub struct DashedIdent(pub Atom);
+
+impl Parse for DashedIdent {
+    fn parse<'i, 't>(_: &ParserContext, input: &mut Parser<'i, 't>) -> Result<Self, ParseError<'i>> {
+        let location = input.current_source_location();
+        let ident = input.expect_ident()?;
+        if ident.starts_with("--") {
+            Ok(Self(Atom::from(ident.as_ref())))
+        } else {
+            Err(location.new_custom_error(SelectorParseErrorKind::UnexpectedIdent(ident.clone())))
+        }
+    }
+}
+
+impl ToCss for DashedIdent {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        serialize_atom_identifier(&self.0, dest)
+    }
+}
+
 /// The <timeline-name> or <keyframes-name>.
 /// The definition of these two names are the same, so we use the same type for them.
 ///
diff --git a/components/style/values/specified/mod.rs b/components/style/values/specified/mod.rs
index b31bc17300e..e157f59d993 100644
--- a/components/style/values/specified/mod.rs
+++ b/components/style/values/specified/mod.rs
@@ -411,6 +411,18 @@ impl NonNegativeNumber {
     }
 }
 
+/// An Integer which is >= 0.
+pub type NonNegativeInteger = NonNegative<Integer>;
+
+impl Parse for NonNegativeInteger {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        Ok(NonNegative(Integer::parse_non_negative(context, input)?))
+    }
+}
+
 /// A Number which is >= 1.0.
 pub type GreaterThanOrEqualToOneNumber = GreaterThanOrEqualToOne<Number>;