Bug 1331213: Bootstrap a Gecko-side Device, and track it's dirtiness manually in the per-doc data. r=heycam

The setup is quite different to Servo-land, so add a comment about the different
setup.

Also, check viewport rules when flushing stylesheets. I believe that the
previous behavior is plain wrong, though I haven't taken the time to come up
with a test case.

In any case, it doesn't hurt any of both back-ends.

MozReview-Commit-ID: 46gtTkesOsr
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

10 files changed, 668 insertions, 66 deletions

diff --git a/components/style/build_gecko.rs b/components/style/build_gecko.rs
index 6bb3b4aeddb..930f751567d 100644
--- a/components/style/build_gecko.rs
+++ b/components/style/build_gecko.rs
@@ -497,6 +497,7 @@ mod bindings {
             "RawGeckoAnimationValueList",
             "RawServoAnimationValue",
             "RawGeckoPresContext",
+            "RawGeckoPresContextOwned",
             "ThreadSafeURIHolder",
             "ThreadSafePrincipalHolder",
             "CSSPseudoClassType",
diff --git a/components/style/gecko/data.rs b/components/style/gecko/data.rs
index dd4ae30a429..f4182d365e2 100644
--- a/components/style/gecko/data.rs
+++ b/components/style/gecko/data.rs
@@ -7,11 +7,10 @@
 use animation::Animation;
 use atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
 use dom::OpaqueNode;
-use euclid::size::TypedSize2D;
-use gecko_bindings::bindings::RawGeckoPresContextBorrowed;
 use gecko_bindings::bindings::RawServoStyleSet;
+use gecko_bindings::structs::RawGeckoPresContextOwned;
 use gecko_bindings::sugar::ownership::{HasBoxFFI, HasFFI, HasSimpleFFI};
-use media_queries::{Device, MediaType};
+use media_queries::Device;
 use num_cpus;
 use parking_lot::RwLock;
 use properties::ComputedValues;
@@ -21,7 +20,6 @@ use std::collections::HashMap;
 use std::env;
 use std::sync::Arc;
 use std::sync::mpsc::{Receiver, Sender, channel};
-use style_traits::ViewportPx;
 use stylesheets::Stylesheet;
 use stylist::Stylist;
 
@@ -37,6 +35,9 @@ pub struct PerDocumentStyleDataImpl {
     /// Whether the stylesheets list above has changed since the last restyle.
     pub stylesheets_changed: bool,
 
+    /// Whether the device has changed since the last restyle.
+    pub device_changed: bool,
+
     // FIXME(bholley): Hook these up to something.
     /// Unused. Will go away when we actually implement transitions and
     /// animations properly.
@@ -57,9 +58,6 @@ pub struct PerDocumentStyleDataImpl {
 
     /// The number of threads of the work queue.
     pub num_threads: usize,
-
-    /// Default computed values for this document.
-    pub default_computed_values: Arc<ComputedValues>
 }
 
 /// The data itself is an `AtomicRefCell`, which guarantees the proper semantics
@@ -78,15 +76,8 @@ lazy_static! {
 
 impl PerDocumentStyleData {
     /// Create a dummy `PerDocumentStyleData`.
-    pub fn new(pres_context: RawGeckoPresContextBorrowed) -> Self {
-        // FIXME(bholley): Real window size.
-        let window_size: TypedSize2D<f32, ViewportPx> = TypedSize2D::new(800.0, 600.0);
-        let default_computed_values = ComputedValues::default_values(pres_context);
-
-        // FIXME(bz): We're going to need to either update the computed values
-        // in the Stylist's Device or give the Stylist a new Device when our
-        // default_computed_values changes.
-        let device = Device::new(MediaType::Screen, window_size, &default_computed_values);
+    pub fn new(pres_context: RawGeckoPresContextOwned) -> Self {
+        let device = Device::new(pres_context);
 
         let (new_anims_sender, new_anims_receiver) = channel();
 
@@ -94,6 +85,7 @@ impl PerDocumentStyleData {
             stylist: Arc::new(Stylist::new(device)),
             stylesheets: vec![],
             stylesheets_changed: true,
+            device_changed: true,
             new_animations_sender: new_anims_sender,
             new_animations_receiver: new_anims_receiver,
             running_animations: Arc::new(RwLock::new(HashMap::new())),
@@ -106,7 +98,6 @@ impl PerDocumentStyleData {
                 rayon::ThreadPool::new(configuration).ok()
             },
             num_threads: *NUM_THREADS,
-            default_computed_values: default_computed_values,
         }))
     }
 
@@ -124,15 +115,30 @@ impl PerDocumentStyleData {
 impl PerDocumentStyleDataImpl {
     /// Recreate the style data if the stylesheets have changed.
     pub fn flush_stylesheets(&mut self) {
-        // The stylist wants to be flushed if either the stylesheets change or the
-        // device dimensions change. When we add support for media queries, we'll
-        // need to detect the latter case and trigger a flush as well.
+        let mut stylist = if self.device_changed || self.stylesheets_changed {
+            Some(Arc::get_mut(&mut self.stylist).unwrap())
+        } else {
+            None
+        };
+
+        if self.device_changed {
+            Arc::get_mut(&mut stylist.as_mut().unwrap().device).unwrap().reset();
+            self.device_changed = false;
+            // Force a stylesheet flush if the device has changed.
+            self.stylesheets_changed = true;
+        }
+
         if self.stylesheets_changed {
-            let _ = Arc::get_mut(&mut self.stylist).unwrap()
-                                                   .update(&self.stylesheets, None, true);
+            let _ = stylist.unwrap().update(&self.stylesheets, None, true);
             self.stylesheets_changed = false;
         }
     }
+
+    /// Get the default computed values for this document.
+    pub fn default_computed_values(&self) -> &Arc<ComputedValues> {
+        debug_assert!(!self.device_changed, "A device flush was pending");
+        self.stylist.device.default_values_arc()
+    }
 }
 
 unsafe impl HasFFI for PerDocumentStyleData {
diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
new file mode 100644
index 00000000000..f7462772d27
--- /dev/null
+++ b/components/style/gecko/media_queries.rs
@@ -0,0 +1,359 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+//! Gecko's media-query device and expression representation.
+
+use app_units::Au;
+use cssparser::{Parser, Token};
+use euclid::Size2D;
+use gecko_bindings::bindings;
+use gecko_bindings::structs::{nsMediaExpression_Range, nsMediaFeature};
+use gecko_bindings::structs::{nsMediaFeature_ValueType, nsMediaFeature_RangeType, nsMediaFeature_RequirementFlags};
+use gecko_bindings::structs::RawGeckoPresContextOwned;
+use media_queries::MediaType;
+use properties::ComputedValues;
+use std::ascii::AsciiExt;
+use std::fmt;
+use std::sync::Arc;
+use string_cache::Atom;
+use style_traits::ToCss;
+use style_traits::viewport::ViewportConstraints;
+use values::{CSSFloat, specified};
+
+/// The `Device` in Gecko wraps a pres context, has a default values computed,
+/// and contains all the viewport rule state.
+pub struct Device {
+    /// NB: The pres context lifetime is tied to the styleset, who owns the
+    /// stylist, and thus the `Device`, so having a raw pres context pointer
+    /// here is fine.
+    pres_context: RawGeckoPresContextOwned,
+    default_values: Arc<ComputedValues>,
+    viewport_override: Option<ViewportConstraints>,
+}
+
+impl Device {
+    /// Trivially constructs a new `Device`.
+    pub fn new(pres_context: RawGeckoPresContextOwned) -> Self {
+        assert!(!pres_context.is_null());
+        Device {
+            pres_context: pres_context,
+            default_values: ComputedValues::default_values(unsafe { &*pres_context }),
+            viewport_override: None,
+        }
+    }
+
+    /// Tells the device that a new viewport rule has been found, and stores the
+    /// relevant viewport constraints.
+    pub fn account_for_viewport_rule(&mut self,
+                                     constraints: &ViewportConstraints) {
+        self.viewport_override = Some(constraints.clone());
+    }
+
+    /// Returns the default computed values as a reference, in order to match
+    /// Servo.
+    pub fn default_values(&self) -> &ComputedValues {
+        &*self.default_values
+    }
+
+    /// Returns the default computed values as an `Arc`, in order to avoid
+    /// clones.
+    pub fn default_values_arc(&self) -> &Arc<ComputedValues> {
+        &self.default_values
+    }
+
+    /// Recreates all the temporary state that the `Device` stores.
+    ///
+    /// This includes the viewport override from `@viewport` rules, and also the
+    /// default computed values.
+    pub fn reset(&mut self) {
+        // NB: A following stylesheet flush will populate this if appropriate.
+        self.viewport_override = None;
+        self.default_values = ComputedValues::default_values(unsafe { &*self.pres_context });
+    }
+
+    /// Returns the current media type of the device.
+    pub fn media_type(&self) -> MediaType {
+        // TODO
+        MediaType::Screen
+    }
+
+    /// Returns the current viewport size in app units.
+    pub fn au_viewport_size(&self) -> Size2D<Au> {
+        self.viewport_override.as_ref().map(|v| {
+            Size2D::new(Au::from_f32_px(v.size.width),
+                        Au::from_f32_px(v.size.height))
+        }).unwrap_or_else(|| {
+            // TODO(emilio): Grab from pres context.
+            Size2D::new(Au::from_f32_px(1024.0),
+                        Au::from_f32_px(768.0))
+        })
+    }
+}
+
+unsafe impl Sync for Device {}
+unsafe impl Send for Device {}
+
+/// A expression for gecko contains a reference to the media feature, the value
+/// the media query contained, and the range to evaluate.
+#[derive(Debug, Clone)]
+pub struct Expression {
+    feature: &'static nsMediaFeature,
+    value: MediaExpressionValue,
+    range: nsMediaExpression_Range
+}
+
+impl ToCss for Expression {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        dest.write_str("(")?;
+        match self.range {
+            nsMediaExpression_Range::eMin => dest.write_str("min-")?,
+            nsMediaExpression_Range::eMax => dest.write_str("max-")?,
+            nsMediaExpression_Range::eEqual => {},
+        }
+        // NB: CSSStringWriter not needed, features are under control.
+        write!(dest, "{}", Atom::from(unsafe { *self.feature.mName }))?;
+        dest.write_str(": ")?;
+
+        self.value.to_css(dest)?;
+        dest.write_str(")")
+    }
+}
+
+/// A resolution.
+#[derive(Debug, Clone)]
+pub enum Resolution {
+    /// Dots per inch.
+    Dpi(CSSFloat),
+    /// Dots per pixel.
+    Dppx(CSSFloat),
+    /// Dots per centimeter.
+    Dpcm(CSSFloat),
+}
+
+impl Resolution {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        let (value, unit) = match try!(input.next()) {
+            Token::Dimension(value, unit) => {
+                (value.value, unit)
+            },
+            _ => return Err(()),
+        };
+
+        Ok(match_ignore_ascii_case! { unit,
+            "dpi" => Resolution::Dpi(value),
+            "dppx" => Resolution::Dppx(value),
+            "dpcm" => Resolution::Dpcm(value),
+            _ => return Err(())
+        })
+    }
+}
+
+impl ToCss for Resolution {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        match *self {
+            Resolution::Dpi(v) => write!(dest, "{}dpi", v),
+            Resolution::Dppx(v) => write!(dest, "{}dppx", v),
+            Resolution::Dpcm(v) => write!(dest, "{}dpcm", v),
+        }
+    }
+}
+
+/// A value found or expected in a media expression.
+#[derive(Debug, Clone)]
+pub enum MediaExpressionValue {
+    /// A length.
+    Length(specified::Length),
+    /// A (non-negative) integer.
+    Integer(u32),
+    /// A floating point value.
+    Float(CSSFloat),
+    /// A boolean value, specified as an integer (i.e., either 0 or 1).
+    BoolInteger(bool),
+    /// Two integers separated by '/', with optional whitespace on either side
+    /// of the '/'.
+    IntRatio(u32, u32),
+    /// A resolution.
+    Resolution(Resolution),
+    /// An enumerated index into the variant keyword table.
+    Enumerated(u32),
+    /// An identifier.
+    Ident(Atom),
+}
+
+impl ToCss for MediaExpressionValue {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        match *self {
+            MediaExpressionValue::Length(ref l) => l.to_css(dest),
+            MediaExpressionValue::Integer(v) => write!(dest, "{}", v),
+            MediaExpressionValue::Float(v) => write!(dest, "{}", v),
+            MediaExpressionValue::BoolInteger(v) => {
+                dest.write_str(if v { "1" } else { "0" })
+            },
+            MediaExpressionValue::IntRatio(a, b) => {
+                write!(dest, "{}/{}", a, b)
+            },
+            MediaExpressionValue::Resolution(ref r) => r.to_css(dest),
+            MediaExpressionValue::Enumerated(..) |
+            MediaExpressionValue::Ident(..) => {
+                // TODO(emilio)
+                unimplemented!()
+            }
+        }
+    }
+}
+
+fn starts_with_ignore_ascii_case(string: &str, prefix: &str) -> bool {
+    string.len() > prefix.len() &&
+      string[0..prefix.len()].eq_ignore_ascii_case(prefix)
+}
+
+#[allow(warnings)]
+fn find_feature<F>(mut f: F) -> Option<&'static nsMediaFeature>
+    where F: FnMut(&'static nsMediaFeature) -> bool,
+{
+    // FIXME(emilio): With build-time bindgen, we would be able to use
+    // structs::nsMediaFeatures_features. That would unfortunately break MSVC
+    // builds, or require one bindings file per platform.
+    //
+    // I'm not into any of those, so meanwhile let's use a FFI function.
+    unsafe {
+        let mut features = bindings::Gecko_GetMediaFeatures();
+        while !(*features).mName.is_null() {
+            if f(&*features) {
+                return Some(&*features);
+            }
+            features = features.offset(1);
+        }
+    }
+
+    None
+}
+
+impl Expression {
+    /// Trivially construct a new expression.
+    fn new(feature: &'static nsMediaFeature,
+           value: MediaExpressionValue,
+           range: nsMediaExpression_Range) -> Self {
+        Expression {
+            feature: feature,
+            value: value,
+            range: range,
+        }
+    }
+
+    /// Parse a media expression of the form:
+    ///
+    /// ```
+    /// (media-feature: media-value)
+    /// ```
+    #[allow(warnings)]
+    pub fn parse(input: &mut Parser) -> Result<Self, ()> {
+        try!(input.expect_parenthesis_block());
+        input.parse_nested_block(|input| {
+            let ident = try!(input.expect_ident());
+            try!(input.expect_colon());
+
+            let mut flags = 0;
+            let mut feature_name = &*ident;
+
+            // TODO(emilio): this is under a pref in Gecko.
+            if starts_with_ignore_ascii_case(feature_name, "-webkit-") {
+                feature_name = &feature_name[8..];
+                flags |= nsMediaFeature_RequirementFlags::eHasWebkitPrefix as u8;
+            }
+
+            let range = if starts_with_ignore_ascii_case(feature_name, "min-") {
+                feature_name = &feature_name[4..];
+                nsMediaExpression_Range::eMin
+            } else if starts_with_ignore_ascii_case(feature_name, "max-") {
+                feature_name = &feature_name[4..];
+                nsMediaExpression_Range::eMax
+            } else {
+                nsMediaExpression_Range::eEqual
+            };
+
+            let atom = Atom::from(feature_name);
+            let feature =
+                match find_feature(|f| atom.as_ptr() == unsafe { *f.mName }) {
+                    Some(f) => f,
+                    None => return Err(()),
+                };
+
+            if (feature.mReqFlags & !flags) != 0 {
+                return Err(());
+            }
+
+            if range != nsMediaExpression_Range::eEqual &&
+                feature.mRangeType != nsMediaFeature_RangeType::eMinMaxAllowed {
+                return Err(());
+            }
+
+            let value = match feature.mValueType {
+                nsMediaFeature_ValueType::eLength => {
+                    MediaExpressionValue::Length(
+                        specified::Length::parse_non_negative(input)?)
+                },
+                nsMediaFeature_ValueType::eInteger => {
+                    let i = input.expect_integer()?;
+                    if i < 0 {
+                        return Err(())
+                    }
+                    MediaExpressionValue::Integer(i as u32)
+                }
+                nsMediaFeature_ValueType::eBoolInteger => {
+                    let i = input.expect_integer()?;
+                    if i < 0 || i > 1 {
+                        return Err(())
+                    }
+                    MediaExpressionValue::BoolInteger(i == 1)
+                }
+                nsMediaFeature_ValueType::eFloat => {
+                    MediaExpressionValue::Float(input.expect_number()?)
+                }
+                nsMediaFeature_ValueType::eIntRatio => {
+                    let a = input.expect_integer()?;
+                    if a <= 0 {
+                        return Err(())
+                    }
+
+                    input.expect_delim('/')?;
+
+                    let b = input.expect_integer()?;
+                    if b <= 0 {
+                        return Err(())
+                    }
+                    MediaExpressionValue::IntRatio(a as u32, b as u32)
+                }
+                nsMediaFeature_ValueType::eResolution => {
+                    MediaExpressionValue::Resolution(Resolution::parse(input)?)
+                }
+                nsMediaFeature_ValueType::eEnumerated => {
+                    let index = unsafe {
+                        let _table = feature.mData.mKeywordTable.as_ref();
+                        0
+                    };
+                    MediaExpressionValue::Enumerated(index)
+                }
+                nsMediaFeature_ValueType::eIdent => {
+                    MediaExpressionValue::Ident(input.expect_ident()?.into())
+                }
+            };
+
+            Ok(Expression::new(feature, value, range))
+        })
+    }
+
+    /// Returns whether this media query evaluates to true for the given
+    /// device.
+    pub fn matches(&self, _device: &Device) -> bool {
+        // TODO
+        false
+    }
+}
diff --git a/components/style/gecko/mod.rs b/components/style/gecko/mod.rs
index 6166a64bea3..ca87d081ef7 100644
--- a/components/style/gecko/mod.rs
+++ b/components/style/gecko/mod.rs
@@ -6,12 +6,7 @@
 
 pub mod conversions;
 pub mod data;
-
-// TODO(emilio): Implement Gecko media query parsing and evaluation using
-// nsMediaFeatures.
-#[path = "../servo/media_queries.rs"]
 pub mod media_queries;
-
 pub mod restyle_damage;
 pub mod selector_parser;
 pub mod snapshot;
diff --git a/components/style/gecko_bindings/bindings.rs b/components/style/gecko_bindings/bindings.rs
index 19635b5af26..89089efca0d 100644
--- a/components/style/gecko_bindings/bindings.rs
+++ b/components/style/gecko_bindings/bindings.rs
@@ -9,6 +9,7 @@ use gecko_bindings::structs::RawGeckoNode;
 use gecko_bindings::structs::RawGeckoAnimationValueList;
 use gecko_bindings::structs::RawServoAnimationValue;
 use gecko_bindings::structs::RawGeckoPresContext;
+use gecko_bindings::structs::RawGeckoPresContextOwned;
 use gecko_bindings::structs::ThreadSafeURIHolder;
 use gecko_bindings::structs::ThreadSafePrincipalHolder;
 use gecko_bindings::structs::CSSPseudoClassType;
@@ -1118,14 +1119,11 @@ extern "C" {
      -> ServoCssRulesStrong;
 }
 extern "C" {
-    pub fn Servo_StyleSet_Init(pres_context: RawGeckoPresContextBorrowed)
+    pub fn Servo_StyleSet_Init(pres_context: RawGeckoPresContextOwned)
      -> RawServoStyleSetOwned;
 }
 extern "C" {
-    pub fn Servo_StyleSet_RecomputeDefaultStyles(set:
-                                                     RawServoStyleSetBorrowed,
-                                                 pres_context:
-                                                     RawGeckoPresContextBorrowed);
+    pub fn Servo_StyleSet_RebuildData(set: RawServoStyleSetBorrowed);
 }
 extern "C" {
     pub fn Servo_StyleSet_AppendStyleSheet(set: RawServoStyleSetBorrowed,
diff --git a/components/style/gecko_bindings/structs_debug.rs b/components/style/gecko_bindings/structs_debug.rs
index 9972511f5e4..619553c2e74 100644
--- a/components/style/gecko_bindings/structs_debug.rs
+++ b/components/style/gecko_bindings/structs_debug.rs
@@ -2957,7 +2957,7 @@ pub mod root {
  * The FramePropertyTable is optimized for storing 0 or 1 properties on
  * a given frame. Storing very large numbers of properties on a single
  * frame will not be efficient.
- * 
+ *
  * Property values are passed as void* but do not actually have to be
  * valid pointers. You can use NS_INT32_TO_PTR/NS_PTR_TO_INT32 to
  * store int32_t values. Null/zero values can be stored and retrieved.
@@ -12149,9 +12149,11 @@ pub mod root {
     pub type RawGeckoElementBorrowedOrNull = *const root::RawGeckoElement;
     pub type RawGeckoDocumentBorrowed = *const root::RawGeckoDocument;
     pub type RawGeckoDocumentBorrowedOrNull = *const root::RawGeckoDocument;
+    pub type RawGeckoPresContextOwned = *mut [u64; 162usize];
     pub type RawGeckoPresContextBorrowed = *const [u64; 162usize];
     pub type RawGeckoAnimationValueListBorrowedMut =
         *mut root::RawGeckoAnimationValueList;
+    pub type RawGeckoPresContextBorrowedMut = *mut [u64; 162usize];
     #[repr(u32)]
     #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
     pub enum nsCSSTokenSerializationType {
@@ -13600,6 +13602,125 @@ pub mod root {
     pub type ThreadSafePrincipalHolder =
         root::nsMainThreadPtrHolder<root::nsIPrincipal>;
     pub type ThreadSafeURIHolder = root::nsMainThreadPtrHolder<root::nsIURI>;
+    #[repr(C)]
+    #[derive(Debug, Copy)]
+    pub struct nsMediaFeature {
+        pub mName: *mut *mut root::nsIAtom,
+        pub mRangeType: root::nsMediaFeature_RangeType,
+        pub mValueType: root::nsMediaFeature_ValueType,
+        pub mReqFlags: u8,
+        pub mData: root::nsMediaFeature__bindgen_ty_1,
+        pub mGetter: root::nsMediaFeatureValueGetter,
+    }
+    #[repr(u32)]
+    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+    pub enum nsMediaFeature_RangeType {
+        eMinMaxAllowed = 0,
+        eMinMaxNotAllowed = 1,
+    }
+    #[repr(u32)]
+    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+    pub enum nsMediaFeature_ValueType {
+        eLength = 0,
+        eInteger = 1,
+        eFloat = 2,
+        eBoolInteger = 3,
+        eIntRatio = 4,
+        eResolution = 5,
+        eEnumerated = 6,
+        eIdent = 7,
+    }
+    #[repr(u8)]
+    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+    pub enum nsMediaFeature_RequirementFlags {
+        eNoRequirements = 0,
+        eHasWebkitPrefix = 1,
+        eWebkitDevicePixelRatioPrefEnabled = 2,
+    }
+    #[repr(C)]
+    #[derive(Debug, Copy)]
+    pub struct nsMediaFeature__bindgen_ty_1 {
+        pub mInitializer_: root::__BindgenUnionField<*const ::std::os::raw::c_void>,
+        pub mKeywordTable: root::__BindgenUnionField<*const root::nsCSSProps_KTableEntry>,
+        pub mMetric: root::__BindgenUnionField<*const *const root::nsIAtom>,
+        pub bindgen_union_field: u64,
+    }
+    #[test]
+    fn bindgen_test_layout_nsMediaFeature__bindgen_ty_1() {
+        assert_eq!(::std::mem::size_of::<nsMediaFeature__bindgen_ty_1>() ,
+                   8usize);
+        assert_eq!(::std::mem::align_of::<nsMediaFeature__bindgen_ty_1>() ,
+                   8usize);
+    }
+    impl Clone for nsMediaFeature__bindgen_ty_1 {
+        fn clone(&self) -> Self { *self }
+    }
+    #[test]
+    fn bindgen_test_layout_nsMediaFeature() {
+        assert_eq!(::std::mem::size_of::<nsMediaFeature>() , 40usize);
+        assert_eq!(::std::mem::align_of::<nsMediaFeature>() , 8usize);
+    }
+    impl Clone for nsMediaFeature {
+        fn clone(&self) -> Self { *self }
+    }
+    pub type ThreadSafePrincipalHolder =
+        root::nsMainThreadPtrHolder<root::nsIPrincipal>;
+    pub type ThreadSafeURIHolder = root::nsMainThreadPtrHolder<root::nsIURI>;
+    pub type nsMediaFeatureValueGetter =
+        ::std::option::Option<unsafe extern "C" fn(aPresContext:
+                                                       *mut root::nsPresContext,
+                                                   aFeature:
+                                                       *const root::nsMediaFeature,
+                                                   aResult:
+                                                       *mut root::nsCSSValue)
+                                  -> root::nsresult>;
+    #[repr(C)]
+    #[derive(Debug, Copy)]
+    pub struct nsMediaFeatures {
+        pub _address: u8,
+    }
+    extern "C" {
+        #[link_name = "_ZN15nsMediaFeatures8featuresE"]
+        pub static mut nsMediaFeatures_features: *const root::nsMediaFeature;
+    }
+    #[test]
+    fn bindgen_test_layout_nsMediaFeatures() {
+        assert_eq!(::std::mem::size_of::<nsMediaFeatures>() , 1usize);
+        assert_eq!(::std::mem::align_of::<nsMediaFeatures>() , 1usize);
+    }
+    impl Clone for nsMediaFeatures {
+        fn clone(&self) -> Self { *self }
+    }
+    #[repr(C)]
+    #[derive(Debug)]
+    pub struct nsMediaExpression {
+        pub mFeature: *const root::nsMediaFeature,
+        pub mRange: root::nsMediaExpression_Range,
+        pub mValue: root::nsCSSValue,
+    }
+    #[repr(u32)]
+    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+    pub enum nsMediaExpression_Range { eMin = 0, eMax = 1, eEqual = 2, }
+    #[test]
+    fn bindgen_test_layout_nsMediaExpression() {
+        assert_eq!(::std::mem::size_of::<nsMediaExpression>() , 32usize);
+        assert_eq!(::std::mem::align_of::<nsMediaExpression>() , 8usize);
+    }
+    #[repr(C)]
+    #[derive(Debug)]
+    pub struct nsMediaQuery {
+        pub mNegated: bool,
+        pub mHasOnly: bool,
+        pub mTypeOmitted: bool,
+        pub mHadUnknownExpression: bool,
+        pub mMediaType: root::nsCOMPtr<root::nsIAtom>,
+        pub mExpressions: root::nsTArray<root::nsMediaExpression>,
+    }
+    #[test]
+    fn bindgen_test_layout_nsMediaQuery() {
+        assert_eq!(::std::mem::size_of::<nsMediaQuery>() , 24usize);
+        assert_eq!(::std::mem::align_of::<nsMediaQuery>() , 8usize);
+    }
     #[test]
     fn __bindgen_test_layout_template_11() {
         assert_eq!(::std::mem::size_of::<root::JS::TenuredHeap<*mut root::JSObject>>()
diff --git a/components/style/gecko_bindings/structs_release.rs b/components/style/gecko_bindings/structs_release.rs
index e7b8fdc5ff2..7d3a1a5caf1 100644
--- a/components/style/gecko_bindings/structs_release.rs
+++ b/components/style/gecko_bindings/structs_release.rs
@@ -2944,7 +2944,7 @@ pub mod root {
  * The FramePropertyTable is optimized for storing 0 or 1 properties on
  * a given frame. Storing very large numbers of properties on a single
  * frame will not be efficient.
- * 
+ *
  * Property values are passed as void* but do not actually have to be
  * valid pointers. You can use NS_INT32_TO_PTR/NS_PTR_TO_INT32 to
  * store int32_t values. Null/zero values can be stored and retrieved.
@@ -12076,9 +12076,11 @@ pub mod root {
     pub type RawGeckoElementBorrowedOrNull = *const root::RawGeckoElement;
     pub type RawGeckoDocumentBorrowed = *const root::RawGeckoDocument;
     pub type RawGeckoDocumentBorrowedOrNull = *const root::RawGeckoDocument;
+    pub type RawGeckoPresContextOwned = *mut [u64; 158usize];
     pub type RawGeckoPresContextBorrowed = *const [u64; 158usize];
     pub type RawGeckoAnimationValueListBorrowedMut =
         *mut root::RawGeckoAnimationValueList;
+    pub type RawGeckoPresContextBorrowedMut = *mut [u64; 158usize];
     #[repr(u32)]
     #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
     pub enum nsCSSTokenSerializationType {
@@ -13524,9 +13526,125 @@ pub mod root {
     pub struct nsTArray<T> {
         pub mBuffer: *mut T,
     }
+    #[repr(C)]
+    #[derive(Debug, Copy)]
+    pub struct nsMediaFeature {
+        pub mName: *mut *mut root::nsIAtom,
+        pub mRangeType: root::nsMediaFeature_RangeType,
+        pub mValueType: root::nsMediaFeature_ValueType,
+        pub mReqFlags: u8,
+        pub mData: root::nsMediaFeature__bindgen_ty_1,
+        pub mGetter: root::nsMediaFeatureValueGetter,
+    }
+    #[repr(u32)]
+    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+    pub enum nsMediaFeature_RangeType {
+        eMinMaxAllowed = 0,
+        eMinMaxNotAllowed = 1,
+    }
+    #[repr(u32)]
+    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+    pub enum nsMediaFeature_ValueType {
+        eLength = 0,
+        eInteger = 1,
+        eFloat = 2,
+        eBoolInteger = 3,
+        eIntRatio = 4,
+        eResolution = 5,
+        eEnumerated = 6,
+        eIdent = 7,
+    }
+    #[repr(u8)]
+    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+    pub enum nsMediaFeature_RequirementFlags {
+        eNoRequirements = 0,
+        eHasWebkitPrefix = 1,
+        eWebkitDevicePixelRatioPrefEnabled = 2,
+    }
+    #[repr(C)]
+    #[derive(Debug, Copy)]
+    pub struct nsMediaFeature__bindgen_ty_1 {
+        pub mInitializer_: root::__BindgenUnionField<*const ::std::os::raw::c_void>,
+        pub mKeywordTable: root::__BindgenUnionField<*const root::nsCSSProps_KTableEntry>,
+        pub mMetric: root::__BindgenUnionField<*const *const root::nsIAtom>,
+        pub bindgen_union_field: u64,
+    }
+    #[test]
+    fn bindgen_test_layout_nsMediaFeature__bindgen_ty_1() {
+        assert_eq!(::std::mem::size_of::<nsMediaFeature__bindgen_ty_1>() ,
+                   8usize);
+        assert_eq!(::std::mem::align_of::<nsMediaFeature__bindgen_ty_1>() ,
+                   8usize);
+    }
+    impl Clone for nsMediaFeature__bindgen_ty_1 {
+        fn clone(&self) -> Self { *self }
+    }
+    #[test]
+    fn bindgen_test_layout_nsMediaFeature() {
+        assert_eq!(::std::mem::size_of::<nsMediaFeature>() , 40usize);
+        assert_eq!(::std::mem::align_of::<nsMediaFeature>() , 8usize);
+    }
+    impl Clone for nsMediaFeature {
+        fn clone(&self) -> Self { *self }
+    }
     pub type ThreadSafePrincipalHolder =
         root::nsMainThreadPtrHolder<root::nsIPrincipal>;
     pub type ThreadSafeURIHolder = root::nsMainThreadPtrHolder<root::nsIURI>;
+    pub type nsMediaFeatureValueGetter =
+        ::std::option::Option<unsafe extern "C" fn(aPresContext:
+                                                       *mut root::nsPresContext,
+                                                   aFeature:
+                                                       *const root::nsMediaFeature,
+                                                   aResult:
+                                                       *mut root::nsCSSValue)
+                                  -> root::nsresult>;
+    #[repr(C)]
+    #[derive(Debug, Copy)]
+    pub struct nsMediaFeatures {
+        pub _address: u8,
+    }
+    extern "C" {
+        #[link_name = "_ZN15nsMediaFeatures8featuresE"]
+        pub static mut nsMediaFeatures_features: *const root::nsMediaFeature;
+    }
+    #[test]
+    fn bindgen_test_layout_nsMediaFeatures() {
+        assert_eq!(::std::mem::size_of::<nsMediaFeatures>() , 1usize);
+        assert_eq!(::std::mem::align_of::<nsMediaFeatures>() , 1usize);
+    }
+    impl Clone for nsMediaFeatures {
+        fn clone(&self) -> Self { *self }
+    }
+    #[repr(C)]
+    #[derive(Debug)]
+    pub struct nsMediaExpression {
+        pub mFeature: *const root::nsMediaFeature,
+        pub mRange: root::nsMediaExpression_Range,
+        pub mValue: root::nsCSSValue,
+    }
+    #[repr(u32)]
+    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+    pub enum nsMediaExpression_Range { eMin = 0, eMax = 1, eEqual = 2, }
+    #[test]
+    fn bindgen_test_layout_nsMediaExpression() {
+        assert_eq!(::std::mem::size_of::<nsMediaExpression>() , 32usize);
+        assert_eq!(::std::mem::align_of::<nsMediaExpression>() , 8usize);
+    }
+    #[repr(C)]
+    #[derive(Debug)]
+    pub struct nsMediaQuery {
+        pub mNegated: bool,
+        pub mHasOnly: bool,
+        pub mTypeOmitted: bool,
+        pub mHadUnknownExpression: bool,
+        pub mMediaType: root::nsCOMPtr<root::nsIAtom>,
+        pub mExpressions: root::nsTArray<root::nsMediaExpression>,
+    }
+    #[test]
+    fn bindgen_test_layout_nsMediaQuery() {
+        assert_eq!(::std::mem::size_of::<nsMediaQuery>() , 24usize);
+        assert_eq!(::std::mem::align_of::<nsMediaQuery>() , 8usize);
+    }
     #[test]
     fn __bindgen_test_layout_template_11() {
         assert_eq!(::std::mem::size_of::<root::JS::TenuredHeap<*mut root::JSObject>>()
diff --git a/components/style/media_queries.rs b/components/style/media_queries.rs
index daac8bed876..19a7fd41b39 100644
--- a/components/style/media_queries.rs
+++ b/components/style/media_queries.rs
@@ -19,7 +19,7 @@ pub use servo::media_queries::{Device, Expression};
 pub use gecko::media_queries::{Device, Expression};
 
 /// A type that encapsulates a media query list.
-#[derive(Debug, PartialEq)]
+#[derive(Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct MediaList {
     /// The list of media queries.
@@ -66,7 +66,7 @@ impl ToCss for Qualifier {
 /// A [media query][mq].
 ///
 /// [mq]: https://drafts.csswg.org/mediaqueries/
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct MediaQuery {
     /// The qualifier for this query.
diff --git a/components/style/servo/media_queries.rs b/components/style/servo/media_queries.rs
index 6dbd78d1587..6c2a4309ddb 100644
--- a/components/style/servo/media_queries.rs
+++ b/components/style/servo/media_queries.rs
@@ -10,8 +10,6 @@ use euclid::{Size2D, TypedSize2D};
 use media_queries::MediaType;
 use properties::ComputedValues;
 use std::fmt;
-#[cfg(feature = "gecko")]
-use std::sync::Arc;
 use style_traits::{ToCss, ViewportPx};
 use style_traits::viewport::ViewportConstraints;
 use values::computed::{self, ToComputedValue};
@@ -21,24 +19,16 @@ use values::specified;
 /// is displayed in.
 ///
 /// This is the struct against which media queries are evaluated.
-#[derive(Debug)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Debug, HeapSizeOf)]
 pub struct Device {
     /// The current media type used by de device.
     media_type: MediaType,
     /// The current viewport size, in viewport pixels.
     viewport_size: TypedSize2D<f32, ViewportPx>,
-    /// A set of default computed values for this document.
-    ///
-    /// This allows handling zoom correctly, among other things. Gecko-only for
-    /// now, see #14773.
-    #[cfg(feature = "gecko")]
-    default_values: Arc<ComputedValues>,
 }
 
 impl Device {
     /// Trivially construct a new `Device`.
-    #[cfg(feature = "servo")]
     pub fn new(media_type: MediaType,
                viewport_size: TypedSize2D<f32, ViewportPx>)
                -> Device {
@@ -48,30 +38,11 @@ impl Device {
         }
     }
 
-    /// Trivially construct a new `Device`.
-    #[cfg(feature = "gecko")]
-    pub fn new(media_type:
-               MediaType, viewport_size: TypedSize2D<f32, ViewportPx>,
-               default_values: &Arc<ComputedValues>) -> Device {
-        Device {
-            media_type: media_type,
-            viewport_size: viewport_size,
-            default_values: default_values.clone(),
-        }
-    }
-
     /// Return the default computed values for this device.
-    #[cfg(feature = "servo")]
     pub fn default_values(&self) -> &ComputedValues {
         ComputedValues::initial_values()
     }
 
-    /// Return the default computed values for this device.
-    #[cfg(feature = "gecko")]
-    pub fn default_values(&self) -> &ComputedValues {
-        &*self.default_values
-    }
-
     /// Returns the viewport size of the current device in app units, needed,
     /// among other things, to resolve viewport units.
     #[inline]
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 45e6b0f9529..27ffea363d1 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -49,6 +49,20 @@ pub use ::fnv::FnvHashMap;
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct Stylist {
     /// Device that the stylist is currently evaluating against.
+    ///
+    /// This field deserves a bigger comment due to the different use that Gecko
+    /// and Servo give to it (that we should eventually unify).
+    ///
+    /// With Gecko, the device is never changed. Gecko manually tracks whether
+    /// the device data should be reconstructed, and "resets" the state of the
+    /// device.
+    ///
+    /// On Servo, on the other hand, the device is a really cheap representation
+    /// that is recreated each time some constraint changes and calling
+    /// `set_device`.
+    ///
+    /// In both cases, the device is actually _owned_ by the Stylist, and it's
+    /// only an `Arc` so we can implement `add_stylesheet` more idiomatically.
     pub device: Arc<Device>,
 
     /// Viewport constraints based on the current device.
@@ -146,6 +160,18 @@ impl Stylist {
             return false;
         }
 
+        let cascaded_rule = ViewportRule {
+            declarations: viewport::Cascade::from_stylesheets(doc_stylesheets, &self.device).finish(),
+        };
+
+        self.viewport_constraints =
+            ViewportConstraints::maybe_new(&self.device, &cascaded_rule);
+
+        if let Some(ref constraints) = self.viewport_constraints {
+            Arc::get_mut(&mut self.device).unwrap()
+                .account_for_viewport_rule(constraints);
+        }
+
         self.element_map = PerPseudoElementSelectorMap::new();
         self.pseudos_map = Default::default();
         self.animations = Default::default();
@@ -394,6 +420,13 @@ impl Stylist {
     ///
     /// Probably worth to make the stylist own a single `Device`, and have a
     /// `update_device` function?
+    ///
+    /// feature = "servo" because gecko only has one device, and manually tracks
+    /// when the device is dirty.
+    ///
+    /// FIXME(emilio): The semantics of the device for Servo and Gecko are
+    /// different enough we may want to unify them.
+    #[cfg(feature = "servo")]
     pub fn set_device(&mut self, mut device: Device, stylesheets: &[Arc<Stylesheet>]) {
         let cascaded_rule = ViewportRule {
             declarations: viewport::Cascade::from_stylesheets(stylesheets, &device).finish(),