[scroll-animations-2] Handle keyframes with offets < 0 or > 1. w3c#7825

Author: fantasai

css-animations-2/Overview.bs

@@ -348,9 +348,13 @@ its associated [=keyframes=] are generated as follows:
 
     1.  Add to the |keyframe properties| of |initial keyframe| and |final keyframe|
         the [=computed value=] on |element|
-        of each property in |animated properties|
-        that is not already assigned to a [=keyframe=] in |keyframes|
-        that has a matching |keyframe offset|.
+        of each property in |animated properties|.
+        Skip any property already declared by a [=keyframe=] in |keyframes|
+        that has a matching |keyframe offset|
+        or that is positioned
+        earlier (for |initial keyframe|)
+        or later (for |final keyframe|)
+        in the [=used keyframe order=].
 
 1.  <strong>Sort Frames:</strong>