Include animations and transitions in the cascade

Instead of applying animations and transitions to styled elements,
include them in the cascade. This allows them to interact properly with
things like font-size and !important rules.

components/style/dom.rs

@@ -479,23 +479,25 @@ pub trait TElement:
     /// Get the combined animation and transition rules.
     ///
     /// FIXME(emilio): Is this really useful?
-    fn animation_rules(&self) -> AnimationRules {
+    fn animation_rules(&self, context: &SharedStyleContext) -> AnimationRules {
         if !self.may_have_animations() {
             return AnimationRules(None, None);
         }
 
-        AnimationRules(self.animation_rule(), self.transition_rule())
+        AnimationRules(self.animation_rule(context), self.transition_rule(context))
     }
 
     /// Get this element's animation rule.
-    fn animation_rule(&self) -> Option<Arc<Locked<PropertyDeclarationBlock>>> {
-        None
-    }
+    fn animation_rule(
+        &self,
+        _: &SharedStyleContext,
+    ) -> Option<Arc<Locked<PropertyDeclarationBlock>>>;
 
     /// Get this element's transition rule.
-    fn transition_rule(&self) -> Option<Arc<Locked<PropertyDeclarationBlock>>> {
-        None
-    }
+    fn transition_rule(
+        &self,
+        context: &SharedStyleContext,
+    ) -> Option<Arc<Locked<PropertyDeclarationBlock>>>;
 
     /// Get this element's state, for non-tree-structural pseudos.
     fn state(&self) -> ElementState;
@@ -729,9 +731,7 @@ pub trait TElement:
     /// In Gecko, element has a flag that represents the element may have
     /// any type of animations or not to bail out animation stuff early.
     /// Whereas Servo doesn't have such flag.
-    fn may_have_animations(&self) -> bool {
-        false
-    }
+    fn may_have_animations(&self) -> bool;
 
     /// Creates a task to update various animation state on a given (pseudo-)element.
     #[cfg(feature = "gecko")]
@@ -748,14 +748,14 @@ pub trait TElement:
     /// Returns true if the element has relevant animations. Relevant
     /// animations are those animations that are affecting the element's style
     /// or are scheduled to do so in the future.
-    fn has_animations(&self) -> bool;
+    fn has_animations(&self, context: &SharedStyleContext) -> bool;
 
     /// Returns true if the element has a CSS animation.
     fn has_css_animations(&self, context: &SharedStyleContext) -> bool;
 
     /// Returns true if the element has a CSS transition (including running transitions and
     /// completed transitions).
-    fn has_css_transitions(&self) -> bool;
+    fn has_css_transitions(&self, context: &SharedStyleContext) -> bool;
 
     /// Returns true if the element has animation restyle hints.
     fn has_animation_restyle_hints(&self) -> bool {