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diff --git a/servo/components/style/gecko/media_queries.rs b/servo/components/style/gecko/media_queries.rs
new file mode 100644
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+++ b/servo/components/style/gecko/media_queries.rs
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+/* 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/. */
+
+//! Gecko's media-query device and expression representation.
+
+use crate::context::QuirksMode;
+use crate::custom_properties::CssEnvironment;
+use crate::gecko::values::{convert_nscolor_to_rgba, convert_rgba_to_nscolor};
+use crate::gecko_bindings::bindings;
+use crate::gecko_bindings::structs;
+use crate::media_queries::MediaType;
+use crate::properties::ComputedValues;
+use crate::string_cache::Atom;
+use crate::values::computed::Length;
+use crate::values::specified::font::FONT_MEDIUM_PX;
+use crate::values::{CustomIdent, KeyframesName};
+use app_units::{Au, AU_PER_PX};
+use cssparser::RGBA;
+use euclid::default::Size2D;
+use euclid::{Scale, SideOffsets2D};
+use servo_arc::Arc;
+use std::sync::atomic::{AtomicBool, AtomicU32, AtomicUsize, Ordering};
+use std::{cmp, fmt};
+use style_traits::viewport::ViewportConstraints;
+use style_traits::{CSSPixel, DevicePixel};
+
+/// 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 document owns the styleset, who owns the stylist, and thus the
+    /// `Device`, so having a raw document pointer here is fine.
+    document: *const structs::Document,
+    default_values: Arc<ComputedValues>,
+    /// The font size of the root element.
+    ///
+    /// This is set when computing the style of the root element, and used for
+    /// rem units in other elements.
+    ///
+    /// When computing the style of the root element, there can't be any other
+    /// style being computed at the same time, given we need the style of the
+    /// parent to compute everything else. So it is correct to just use a
+    /// relaxed atomic here.
+    root_font_size: AtomicU32,
+    /// The body text color, stored as an `nscolor`, used for the "tables
+    /// inherit from body" quirk.
+    ///
+    /// <https://quirks.spec.whatwg.org/#the-tables-inherit-color-from-body-quirk>
+    body_text_color: AtomicUsize,
+    /// Whether any styles computed in the document relied on the root font-size
+    /// by using rem units.
+    used_root_font_size: AtomicBool,
+    /// Whether any styles computed in the document relied on the viewport size
+    /// by using vw/vh/vmin/vmax units.
+    used_viewport_size: AtomicBool,
+    /// The CssEnvironment object responsible of getting CSS environment
+    /// variables.
+    environment: CssEnvironment,
+}
+
+impl fmt::Debug for Device {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use nsstring::nsCString;
+
+        let mut doc_uri = nsCString::new();
+        unsafe {
+            bindings::Gecko_nsIURI_Debug(
+                (*self.document()).mDocumentURI.raw::<structs::nsIURI>(),
+                &mut doc_uri,
+            )
+        };
+
+        f.debug_struct("Device")
+            .field("document_url", &doc_uri)
+            .finish()
+    }
+}
+
+unsafe impl Sync for Device {}
+unsafe impl Send for Device {}
+
+impl Device {
+    /// Trivially constructs a new `Device`.
+    pub fn new(document: *const structs::Document) -> Self {
+        assert!(!document.is_null());
+        let doc = unsafe { &*document };
+        let prefs = unsafe { &*bindings::Gecko_GetPrefSheetPrefs(doc) };
+        Device {
+            document,
+            default_values: ComputedValues::default_values(doc),
+            root_font_size: AtomicU32::new(FONT_MEDIUM_PX.to_bits()),
+            body_text_color: AtomicUsize::new(prefs.mDefaultColor as usize),
+            used_root_font_size: AtomicBool::new(false),
+            used_viewport_size: AtomicBool::new(false),
+            environment: CssEnvironment,
+        }
+    }
+
+    /// Get the relevant environment to resolve `env()` functions.
+    #[inline]
+    pub fn environment(&self) -> &CssEnvironment {
+        &self.environment
+    }
+
+    /// 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) {
+        unreachable!("Gecko doesn't support @viewport");
+    }
+
+    /// Whether any animation name may be referenced from the style of any
+    /// element.
+    pub fn animation_name_may_be_referenced(&self, name: &KeyframesName) -> bool {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return false,
+        };
+
+        unsafe {
+            bindings::Gecko_AnimationNameMayBeReferencedFromStyle(pc, name.as_atom().as_ptr())
+        }
+    }
+
+    /// Returns the default computed values as a reference, in order to match
+    /// Servo.
+    pub fn default_computed_values(&self) -> &ComputedValues {
+        &self.default_values
+    }
+
+    /// Returns the default computed values as an `Arc`.
+    pub fn default_computed_values_arc(&self) -> &Arc<ComputedValues> {
+        &self.default_values
+    }
+
+    /// Get the font size of the root element (for rem)
+    pub fn root_font_size(&self) -> Length {
+        self.used_root_font_size.store(true, Ordering::Relaxed);
+        Length::new(f32::from_bits(self.root_font_size.load(Ordering::Relaxed)))
+    }
+
+    /// Set the font size of the root element (for rem)
+    pub fn set_root_font_size(&self, size: Length) {
+        self.root_font_size
+            .store(size.px().to_bits(), Ordering::Relaxed)
+    }
+
+    /// The quirks mode of the document.
+    pub fn quirks_mode(&self) -> QuirksMode {
+        self.document().mCompatMode.into()
+    }
+
+    /// Sets the body text color for the "inherit color from body" quirk.
+    ///
+    /// <https://quirks.spec.whatwg.org/#the-tables-inherit-color-from-body-quirk>
+    pub fn set_body_text_color(&self, color: RGBA) {
+        self.body_text_color
+            .store(convert_rgba_to_nscolor(&color) as usize, Ordering::Relaxed)
+    }
+
+    /// Returns the body text color.
+    pub fn body_text_color(&self) -> RGBA {
+        convert_nscolor_to_rgba(self.body_text_color.load(Ordering::Relaxed) as u32)
+    }
+
+    /// Gets the document pointer.
+    #[inline]
+    pub fn document(&self) -> &structs::Document {
+        unsafe { &*self.document }
+    }
+
+    /// Gets the pres context associated with this document.
+    #[inline]
+    pub fn pres_context(&self) -> Option<&structs::nsPresContext> {
+        unsafe {
+            self.document()
+                .mPresShell
+                .as_ref()?
+                .mPresContext
+                .mRawPtr
+                .as_ref()
+        }
+    }
+
+    /// Gets the preference stylesheet prefs for our document.
+    #[inline]
+    pub fn pref_sheet_prefs(&self) -> &structs::PreferenceSheet_Prefs {
+        unsafe { &*bindings::Gecko_GetPrefSheetPrefs(self.document()) }
+    }
+
+    /// Recreates the default computed values.
+    pub fn reset_computed_values(&mut self) {
+        self.default_values = ComputedValues::default_values(self.document());
+    }
+
+    /// Rebuild all the cached data.
+    pub fn rebuild_cached_data(&mut self) {
+        self.reset_computed_values();
+        self.used_root_font_size.store(false, Ordering::Relaxed);
+        self.used_viewport_size.store(false, Ordering::Relaxed);
+    }
+
+    /// Returns whether we ever looked up the root font size of the Device.
+    pub fn used_root_font_size(&self) -> bool {
+        self.used_root_font_size.load(Ordering::Relaxed)
+    }
+
+    /// 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) {
+        self.reset_computed_values();
+    }
+
+    /// Returns whether this document is in print preview.
+    pub fn is_print_preview(&self) -> bool {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return false,
+        };
+        pc.mType == structs::nsPresContext_nsPresContextType_eContext_PrintPreview
+    }
+
+    /// Returns the current media type of the device.
+    pub fn media_type(&self) -> MediaType {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return MediaType::screen(),
+        };
+
+        // Gecko allows emulating random media with mMediaEmulationData.mMedium.
+        let medium_to_use = if !pc.mMediaEmulationData.mMedium.mRawPtr.is_null() {
+            pc.mMediaEmulationData.mMedium.mRawPtr
+        } else {
+            pc.mMedium as *const structs::nsAtom as *mut _
+        };
+
+        MediaType(CustomIdent(unsafe { Atom::from_raw(medium_to_use) }))
+    }
+
+    // It may make sense to account for @page rule margins here somehow, however
+    // it's not clear how that'd work, see:
+    // https://github.com/w3c/csswg-drafts/issues/5437
+    fn page_size_minus_default_margin(&self, pc: &structs::nsPresContext) -> Size2D<Au> {
+        debug_assert!(pc.mIsRootPaginatedDocument() != 0);
+        let area = &pc.mPageSize;
+        let margin = &pc.mDefaultPageMargin;
+        let width = area.width - margin.left - margin.right;
+        let height = area.height - margin.top - margin.bottom;
+        Size2D::new(Au(cmp::max(width, 0)), Au(cmp::max(height, 0)))
+    }
+
+    /// Returns the current viewport size in app units.
+    pub fn au_viewport_size(&self) -> Size2D<Au> {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return Size2D::new(Au(0), Au(0)),
+        };
+
+        if pc.mIsRootPaginatedDocument() != 0 {
+            return self.page_size_minus_default_margin(pc);
+        }
+
+        let area = &pc.mVisibleArea;
+        Size2D::new(Au(area.width), Au(area.height))
+    }
+
+    /// Returns the current viewport size in app units, recording that it's been
+    /// used for viewport unit resolution.
+    pub fn au_viewport_size_for_viewport_unit_resolution(&self) -> Size2D<Au> {
+        self.used_viewport_size.store(true, Ordering::Relaxed);
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return Size2D::new(Au(0), Au(0)),
+        };
+
+        if pc.mIsRootPaginatedDocument() != 0 {
+            return self.page_size_minus_default_margin(pc);
+        }
+
+        let size = &pc.mSizeForViewportUnits;
+        Size2D::new(Au(size.width), Au(size.height))
+    }
+
+    /// Returns whether we ever looked up the viewport size of the Device.
+    pub fn used_viewport_size(&self) -> bool {
+        self.used_viewport_size.load(Ordering::Relaxed)
+    }
+
+    /// Returns the device pixel ratio.
+    pub fn device_pixel_ratio(&self) -> Scale<f32, CSSPixel, DevicePixel> {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return Scale::new(1.),
+        };
+
+        if pc.mMediaEmulationData.mDPPX > 0.0 {
+            return Scale::new(pc.mMediaEmulationData.mDPPX);
+        }
+
+        let au_per_dpx = pc.mCurAppUnitsPerDevPixel as f32;
+        let au_per_px = AU_PER_PX as f32;
+        Scale::new(au_per_px / au_per_dpx)
+    }
+
+    /// Returns whether document colors are enabled.
+    #[inline]
+    pub fn use_document_colors(&self) -> bool {
+        let doc = self.document();
+        if doc.mIsBeingUsedAsImage() {
+            return true;
+        }
+        self.pref_sheet_prefs().mUseDocumentColors
+    }
+
+    /// Returns the default background color.
+    pub fn default_background_color(&self) -> RGBA {
+        convert_nscolor_to_rgba(self.pref_sheet_prefs().mDefaultBackgroundColor)
+    }
+
+    /// Returns the default foreground color.
+    pub fn default_color(&self) -> RGBA {
+        convert_nscolor_to_rgba(self.pref_sheet_prefs().mDefaultColor)
+    }
+
+    /// Returns the current effective text zoom.
+    #[inline]
+    fn effective_text_zoom(&self) -> f32 {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return 1.,
+        };
+        pc.mEffectiveTextZoom
+    }
+
+    /// Applies text zoom to a font-size or line-height value (see nsStyleFont::ZoomText).
+    #[inline]
+    pub fn zoom_text(&self, size: Length) -> Length {
+        size.scale_by(self.effective_text_zoom())
+    }
+
+    /// Un-apply text zoom.
+    #[inline]
+    pub fn unzoom_text(&self, size: Length) -> Length {
+        size.scale_by(1. / self.effective_text_zoom())
+    }
+
+    /// Returns safe area insets
+    pub fn safe_area_insets(&self) -> SideOffsets2D<f32, CSSPixel> {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return SideOffsets2D::zero(),
+        };
+        let mut top = 0.0;
+        let mut right = 0.0;
+        let mut bottom = 0.0;
+        let mut left = 0.0;
+        unsafe {
+            bindings::Gecko_GetSafeAreaInsets(pc, &mut top, &mut right, &mut bottom, &mut left)
+        };
+        SideOffsets2D::new(top, right, bottom, left)
+    }
+}