[css-animations-2] Sort percentage keyframes to the front of the list. w3c#8507

Author: fantasai

css-animations-2/Overview.bs

@@ -361,8 +361,15 @@ its associated [=keyframes=] are generated as follows:
 
     *   The <dfn lt="computed keyframe order">computed order</dfn> of |keyframes|--
         which is the order returned by {{KeyframeEffect/getKeyframes()}}--
-        is found by performing a stable sort on |keyframes|
-        by each keyframe’s |keyframe offset|.
+        is found by
+        shifting |initial keyframe|, |final keyframe|, and
+        any keyframes whose offset was specified
+        as <<percentage>>, ''@keyframes/from'', or ''@keyframes/to''
+        to the front of the list,
+        and performing a stable sort on these keyframes
+        by their |keyframe offset|s.
+
+        ISSUE: It might be more natural to leave |final keyframe| at the end.
 
     *   The <dfn lt="used keyframe order">used order</dfn> of |keyframes|--
         which is the order used to interpolate and compute the actual animation frames--
@@ -372,6 +379,13 @@ its associated [=keyframes=] are generated as follows:
         and ordering them from earliest to latest,
         breaking ties by using the [=computed keyframe order=].
 
+    Note: Although the [=computed keyframe order=]
+    sorts keyframes with <<percentage>> offsets,
+    it maintains keyframes specified with a <<timeline-range-name>>
+    in their [=specified keyframe order=]--
+    after any <<percentage>> keyframes,
+    even if these come later in the [=used keyframe order=].
+
 <h2 id="animation-definition">
 Declaring Animations</h2>