diff --git a/components/layout/animation.rs b/components/layout/animation.rs
index ebde01323e5..9b1d0671b9a 100644
--- a/components/layout/animation.rs
+++ b/components/layout/animation.rs
@@ -26,7 +26,7 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
                               script_chan: &IpcSender<ConstellationControlMsg>,
                               running_animations: &mut HashMap<OpaqueNode, Vec<Animation>>,
                               expired_animations: &mut HashMap<OpaqueNode, Vec<Animation>>,
-                              newly_transitioning_nodes: &mut Vec<UntrustedNodeAddress>,
+                              mut newly_transitioning_nodes: Option<&mut Vec<UntrustedNodeAddress>>,
                               new_animations_receiver: &Receiver<Animation>,
                               pipeline_id: PipelineId,
                               timer: &Timer) {
@@ -115,8 +115,11 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
 
     // Add new running animations.
     for new_running_animation in new_running_animations {
-        if new_running_animation.is_transition() {
-            newly_transitioning_nodes.push(new_running_animation.node().to_untrusted_node_address());
+        match newly_transitioning_nodes {
+            Some(ref mut nodes) if new_running_animation.is_transition() => {
+                nodes.push(new_running_animation.node().to_untrusted_node_address());
+            }
+            _ => ()
         }
 
         running_animations.entry(*new_running_animation.node())
diff --git a/components/layout/context.rs b/components/layout/context.rs
index 214061e276f..8acbc3b254a 100644
--- a/components/layout/context.rs
+++ b/components/layout/context.rs
@@ -15,6 +15,7 @@ use net_traits::image_cache::{ImageOrMetadataAvailable, UsePlaceholder};
 use opaque_node::OpaqueNodeMethods;
 use parking_lot::RwLock;
 use script_layout_interface::{PendingImage, PendingImageState};
+use script_traits::UntrustedNodeAddress;
 use servo_url::ServoUrl;
 use std::borrow::{Borrow, BorrowMut};
 use std::cell::{RefCell, RefMut};
@@ -96,7 +97,11 @@ pub struct LayoutContext<'a> {
 
     /// A list of in-progress image loads to be shared with the script thread.
     /// A None value means that this layout was not initiated by the script thread.
-    pub pending_images: Option<Mutex<Vec<PendingImage>>>
+    pub pending_images: Option<Mutex<Vec<PendingImage>>>,
+
+    /// A list of nodes that have just initiated a CSS transition.
+    /// A None value means that this layout was not initiated by the script thread.
+    pub newly_transitioning_nodes: Option<Mutex<Vec<UntrustedNodeAddress>>>,
 }
 
 impl<'a> Drop for LayoutContext<'a> {
diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index 2bef4bc1c21..32b47d57a22 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -514,7 +514,7 @@ impl LayoutThread {
     // Create a layout context for use in building display lists, hit testing, &c.
     fn build_layout_context<'a>(&'a self,
                                 guards: StylesheetGuards<'a>,
-                                request_images: bool,
+                                script_initiated_layout: bool,
                                 snapshot_map: &'a SnapshotMap)
                                 -> LayoutContext<'a> {
         let thread_local_style_context_creation_data =
@@ -538,7 +538,8 @@ impl LayoutThread {
             image_cache: self.image_cache.clone(),
             font_cache_thread: Mutex::new(self.font_cache_thread.clone()),
             webrender_image_cache: self.webrender_image_cache.clone(),
-            pending_images: if request_images { Some(Mutex::new(vec![])) } else { None },
+            pending_images: if script_initiated_layout { Some(Mutex::new(vec![])) } else { None },
+            newly_transitioning_nodes: if script_initiated_layout { Some(Mutex::new(vec![])) } else { None },
         }
     }
 
@@ -1252,6 +1253,12 @@ impl LayoutThread {
         };
         rw_data.pending_images = pending_images;
 
+        let newly_transitioning_nodes = match context.newly_transitioning_nodes {
+            Some(ref nodes) => std_mem::replace(&mut *nodes.lock().unwrap(), vec![]),
+            None => vec![],
+        };
+        rw_data.newly_transitioning_nodes = newly_transitioning_nodes;
+
         let mut root_flow = match self.root_flow.borrow().clone() {
             Some(root_flow) => root_flow,
             None => return,
@@ -1427,6 +1434,7 @@ impl LayoutThread {
                                                          &mut *rw_data,
                                                          &mut layout_context);
             assert!(layout_context.pending_images.is_none());
+            assert!(layout_context.newly_transitioning_nodes.is_none());
         }
     }
 
@@ -1437,17 +1445,24 @@ impl LayoutThread {
                                                document: Option<&ServoLayoutDocument>,
                                                rw_data: &mut LayoutThreadData,
                                                context: &mut LayoutContext) {
-        assert!(rw_data.newly_transitioning_nodes.is_empty());
-
-        // Kick off animations if any were triggered, expire completed ones.
-        animation::update_animation_state(&self.constellation_chan,
-                                          &self.script_chan,
-                                          &mut *self.running_animations.write(),
-                                          &mut *self.expired_animations.write(),
-                                          &mut rw_data.newly_transitioning_nodes,
-                                          &self.new_animations_receiver,
-                                          self.id,
-                                          &self.timer);
+        {
+            let mut newly_transitioning_nodes = context
+                .newly_transitioning_nodes
+                .as_ref()
+                .map(|nodes| nodes.lock().unwrap());
+            let newly_transitioning_nodes = newly_transitioning_nodes
+                .as_mut()
+                .map(|nodes| &mut **nodes);
+            // Kick off animations if any were triggered, expire completed ones.
+            animation::update_animation_state(&self.constellation_chan,
+                                              &self.script_chan,
+                                              &mut *self.running_animations.write(),
+                                              &mut *self.expired_animations.write(),
+                                              newly_transitioning_nodes,
+                                              &self.new_animations_receiver,
+                                              self.id,
+                                              &self.timer);
+        }
 
         profile(time::ProfilerCategory::LayoutRestyleDamagePropagation,
                 self.profiler_metadata(),