Auto merge of #17839 - heycam:opacity-smil, r=hiro

style: allow out-of-range opacity values for SMIL animations

From https://bugzilla.mozilla.org/show_bug.cgi?id=1371150.

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components/style/gecko/media_queries.rs

@@ -650,6 +650,7 @@ impl Expression {
             in_media_query: true,
             // TODO: pass the correct value here.
             quirks_mode: quirks_mode,
+            for_smil_animation: false,
         };
 
         let required_value = match self.value {

components/style/properties/properties.mako.rs

@@ -2996,6 +2996,7 @@ where
         cached_system_font: None,
         in_media_query: false,
         quirks_mode: quirks_mode,
+        for_smil_animation: false,
     };
 
     let ignore_colors = !device.use_document_colors();

components/style/servo/media_queries.rs

@@ -246,6 +246,7 @@ impl Range<specified::Length> {
             in_media_query: true,
             cached_system_font: None,
             quirks_mode: quirks_mode,
+            for_smil_animation: false,
         };
 
         match *self {

components/style/stylesheets/viewport_rule.rs

@@ -709,6 +709,7 @@ impl MaybeNew for ViewportConstraints {
             cached_system_font: None,
             in_media_query: false,
             quirks_mode: quirks_mode,
+            for_smil_animation: false,
         };
 
         // DEVICE-ADAPT § 9.3 Resolving 'extend-to-zoom'

components/style/values/computed/mod.rs

@@ -95,6 +95,12 @@ pub struct Context<'a> {
 
     /// The quirks mode of this context.
     pub quirks_mode: QuirksMode,
+
+    /// Whether this computation is being done for a SMIL animation.
+    ///
+    /// This is used to allow certain properties to generate out-of-range
+    /// values, which SMIL allows.
+    pub for_smil_animation: bool,
 }
 
 impl<'a> Context<'a> {

components/style/values/specified/mod.rs

@@ -566,7 +566,14 @@ impl ToComputedValue for Opacity {
 
     #[inline]
     fn to_computed_value(&self, context: &Context) -> CSSFloat {
-        self.0.to_computed_value(context).min(1.0).max(0.0)
+        let value = self.0.to_computed_value(context);
+        if context.for_smil_animation {
+            // SMIL expects to be able to interpolate between out-of-range
+            // opacity values.
+            value
+        } else {
+            value.min(1.0).max(0.0)
+        }
     }
 
     #[inline]