style: Implement the prefers-contrast media-query.

Differential Revision: https://phabricator.services.mozilla.com/D79553

components/style/gecko/media_features.rs

@@ -308,6 +308,53 @@ fn eval_prefers_reduced_motion(device: &Device, query_value: Option<PrefersReduc
     }
 }
 
+/// Possible values for prefers-contrast media query.
+/// https://drafts.csswg.org/mediaqueries-5/#prefers-contrast
+#[derive(Clone, Copy, Debug, FromPrimitive, PartialEq, Parse, ToCss)]
+#[repr(u8)]
+#[allow(missing_docs)]
+enum PrefersContrast {
+    High,
+    Low,
+    NoPreference,
+    Forced,
+}
+
+/// Represents the parts of prefers-contrast that explicitly deal with
+/// contrast. Used in combination with information about rather or not
+/// forced colors are active this allows for evaluation of the
+/// prefers-contrast media query.
+#[derive(Clone, Copy, Debug, FromPrimitive, PartialEq)]
+#[repr(u8)]
+pub enum ContrastPref {
+    /// High contrast is prefered. Corresponds to an accessibility theme
+    /// being enabled or firefox forcing high contrast colors.
+    High,
+    /// Low contrast is prefered. Corresponds to the
+    /// browser.display.prefers_low_contrast pref being true.
+    Low,
+    /// The default value if neither high or low contrast is enabled.
+    NoPreference,
+}
+
+/// https://drafts.csswg.org/mediaqueries-5/#prefers-contrast
+fn eval_prefers_contrast(device: &Device, query_value: Option<PrefersContrast>) -> bool {
+    let forced_colors = !device.use_document_colors();
+    let contrast_pref =
+        unsafe { bindings::Gecko_MediaFeatures_PrefersContrast(device.document(), forced_colors) };
+    if let Some(query_value) = query_value {
+        match query_value {
+            PrefersContrast::Forced => forced_colors,
+            PrefersContrast::High => contrast_pref == ContrastPref::High,
+            PrefersContrast::Low => contrast_pref == ContrastPref::Low,
+            PrefersContrast::NoPreference => contrast_pref == ContrastPref::NoPreference,
+        }
+    } else {
+        // Only prefers-contrast: no-preference evaluates to false.
+        forced_colors || (contrast_pref != ContrastPref::NoPreference)
+    }
+}
+
 #[derive(Clone, Copy, Debug, FromPrimitive, Parse, ToCss)]
 #[repr(u8)]
 enum OverflowBlock {
@@ -548,7 +595,7 @@ macro_rules! system_metric_feature {
 /// to support new types in these entries and (2) ensuring that either
 /// nsPresContext::MediaFeatureValuesChanged is called when the value that
 /// would be returned by the evaluator function could change.
-pub static MEDIA_FEATURES: [MediaFeatureDescription; 53] = [
+pub static MEDIA_FEATURES: [MediaFeatureDescription; 54] = [
     feature!(
         atom!("width"),
         AllowsRanges::Yes,
@@ -666,6 +713,17 @@ pub static MEDIA_FEATURES: [MediaFeatureDescription; 53] = [
         keyword_evaluator!(eval_prefers_reduced_motion, PrefersReducedMotion),
         ParsingRequirements::empty(),
     ),
+    feature!(
+        atom!("prefers-contrast"),
+        AllowsRanges::No,
+        keyword_evaluator!(eval_prefers_contrast, PrefersContrast),
+        // Note: by default this is only enabled in browser chrome and
+        // ua. It can be enabled on the web via the
+        // layout.css.prefers-contrast.enabled preference. See
+        // disabed_by_pref in media_feature_expression.rs for how that
+        // is done.
+        ParsingRequirements::empty(),
+    ),
     feature!(
         atom!("overflow-block"),
         AllowsRanges::No,

components/style/media_queries/media_feature_expression.rs

@@ -218,12 +218,18 @@ fn consume_operation_or_colon(input: &mut Parser) -> Result<Option<Operator>, ()
 }
 
 #[allow(unused_variables)]
-fn disabled_by_pref(feature: &Atom) -> bool {
+fn disabled_by_pref(feature: &Atom, context: &ParserContext) -> bool {
     #[cfg(feature = "gecko")]
     {
         if *feature == atom!("-moz-touch-enabled") {
             return !static_prefs::pref!("layout.css.moz-touch-enabled.enabled");
         }
+        // prefers-contrast is always enabled in the ua and chrome. On
+        // the web it is hidden behind a preference.
+        if *feature == atom!("prefers-contrast") {
+            return !context.in_ua_or_chrome_sheet() &&
+                !static_prefs::pref!("layout.css.prefers-contrast.enabled");
+        }
     }
     false
 }
@@ -305,7 +311,7 @@ impl MediaFeatureExpression {
             },
         };
 
-        if disabled_by_pref(&feature.name) ||
+        if disabled_by_pref(&feature.name, context) ||
             !requirements.contains(feature.requirements) ||
             (range.is_some() && !feature.allows_ranges())
         {

components/style/properties/gecko.mako.rs

@@ -2089,7 +2089,9 @@ pub fn assert_initial_values_match(data: &PerDocumentStyleData) {
         let data = data.borrow();
         let cv = data.stylist.device().default_computed_values();
         <%
-            # Skip properties with initial values that change at computed value time.
+            # Skip properties with initial values that change at computed
+            # value time, or whose initial value depends on the document
+            # / other prefs.
             SKIPPED = [
                 "border-top-width",
                 "border-bottom-width",
@@ -2098,6 +2100,7 @@ pub fn assert_initial_values_match(data: &PerDocumentStyleData) {
                 "font-family",
                 "font-size",
                 "outline-width",
+                "color",
             ]
             TO_TEST = [p for p in data.longhands if p.enabled_in != "" and not p.logical and not p.name in SKIPPED]
         %>