Add animation and transition support for pseudo-elements

This change extends the DocumentAnimationSet to hold animations for
pseudo-elements. Since pseudo-elements in Servo are not in the DOM like
in Gecko, they need to be handled a bit carefully in stylo.  When a
pseudo-element has an animation, recascade the style. Finally, this
change passes the pseudo-element string properly to animation events.

Fixes: #10316

components/style/style_resolver.rs

@@ -118,6 +118,17 @@ where
     )
 }
 
+fn layout_parent_style_for_pseudo<'a>(
+    primary_style: &'a PrimaryStyle,
+    layout_parent_style: Option<&'a ComputedValues>,
+) -> Option<&'a ComputedValues> {
+    if primary_style.style().is_display_contents() {
+        layout_parent_style
+    } else {
+        Some(primary_style.style())
+    }
+}
+
 fn eager_pseudo_is_definitely_not_generated(
     pseudo: &PseudoElement,
     style: &ComputedValues,
@@ -246,11 +257,8 @@ where
         let mut pseudo_styles = EagerPseudoStyles::default();
 
         if !self.element.is_pseudo_element() {
-            let layout_parent_style_for_pseudo = if primary_style.style().is_display_contents() {
-                layout_parent_style
-            } else {
-                Some(primary_style.style())
-            };
+            let layout_parent_style_for_pseudo =
+                layout_parent_style_for_pseudo(&primary_style, layout_parent_style);
             SelectorImpl::each_eagerly_cascaded_pseudo_element(|pseudo| {
                 let pseudo_style = self.resolve_pseudo_style(
                     &pseudo,
@@ -298,6 +306,26 @@ where
         })
     }
 
+    /// Cascade a set of rules for pseudo element, using the default parent for inheritance.
+    pub(crate) fn cascade_style_and_visited_for_pseudo_with_default_parents(
+        &mut self,
+        inputs: CascadeInputs,
+        pseudo: &PseudoElement,
+        primary_style: &PrimaryStyle,
+    ) -> ResolvedStyle {
+        with_default_parent_styles(self.element, |_, layout_parent_style| {
+            let layout_parent_style_for_pseudo =
+                layout_parent_style_for_pseudo(primary_style, layout_parent_style);
+
+            self.cascade_style_and_visited(
+                inputs,
+                Some(primary_style.style()),
+                layout_parent_style_for_pseudo,
+                Some(pseudo),
+            )
+        })
+    }
+
     fn cascade_style_and_visited(
         &mut self,
         inputs: CascadeInputs,