Auto merge of #26806 - mrobinson:animations-cascade, r=emilio

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.

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---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

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components/layout_thread_2020/dom_wrapper.rs

@@ -472,6 +472,54 @@ impl<'le> TElement for ServoLayoutElement<'le> {
         }
     }
 
+    fn may_have_animations(&self) -> bool {
+        true
+    }
+
+    fn animation_rule(
+        &self,
+        context: &SharedStyleContext,
+    ) -> Option<Arc<StyleLocked<PropertyDeclarationBlock>>> {
+        let node = self.as_node();
+        let document = node.owner_doc();
+        context
+            .animation_states
+            .read()
+            .get(&node.opaque())
+            .and_then(|set| {
+                set.get_value_map_for_active_animations(context.current_time_for_animations)
+            })
+            .map(|map| {
+                Arc::new(
+                    document
+                        .style_shared_lock()
+                        .wrap(PropertyDeclarationBlock::from_animation_value_map(&map)),
+                )
+            })
+    }
+
+    fn transition_rule(
+        &self,
+        context: &SharedStyleContext,
+    ) -> Option<Arc<StyleLocked<PropertyDeclarationBlock>>> {
+        let node = self.as_node();
+        let document = node.owner_doc();
+        context
+            .animation_states
+            .read()
+            .get(&node.opaque())
+            .and_then(|set| {
+                set.get_value_map_for_active_transitions(context.current_time_for_animations)
+            })
+            .map(|map| {
+                Arc::new(
+                    document
+                        .style_shared_lock()
+                        .wrap(PropertyDeclarationBlock::from_animation_value_map(&map)),
+                )
+            })
+    }
+
     fn state(&self) -> ElementState {
         self.element.get_state_for_layout()
     }
@@ -589,11 +637,8 @@ impl<'le> TElement for ServoLayoutElement<'le> {
         self.element.has_selector_flags(flags)
     }
 
-    fn has_animations(&self) -> bool {
-        // We use this function not only for Gecko but also for Servo to know if this element has
-        // animations, so we maybe try to get the important rules of this element. This is used for
-        // off-main thread animations, but we don't support it on Servo, so return false directly.
-        false
+    fn has_animations(&self, context: &SharedStyleContext) -> bool {
+        return self.has_css_animations(context) || self.has_css_transitions(context);
     }
 
     fn has_css_animations(&self, context: &SharedStyleContext) -> bool {
@@ -605,8 +650,13 @@ impl<'le> TElement for ServoLayoutElement<'le> {
             .unwrap_or(false)
     }
 
-    fn has_css_transitions(&self) -> bool {
-        unreachable!("this should be only called on gecko");
+    fn has_css_transitions(&self, context: &SharedStyleContext) -> bool {
+        context
+            .animation_states
+            .read()
+            .get(&self.as_node().opaque())
+            .map(|set| set.has_active_transition())
+            .unwrap_or(false)
     }
 
     #[inline]