[scroll-animations-2] Don't generate initial/final keyframes unless necessary.

Author: fantasai

css-animations-2/Overview.bs

@@ -337,24 +337,24 @@ its associated [=keyframes=] are generated as follows:
 
 1.  <strong>Generate Initial and Final Frames:</strong>
 
-    1.  Create two new [=keyframes=], |initial keyframe| and |final keyframe|,
-        assigning each |default timing function| as its |keyframe timing function|
+    1.  Find or create the |initial keyframe|,
+        a [=keyframe=] with a |keyframe offset| of ''0%'',
+        |default timing function| as its |keyframe timing function|,
         and |default composite| as its |keyframe composite|.
 
-    1.  Give |initial keyframe| a |keyframe offset| of zero (''0%'')
-        and prepend it to |keyframes|;
-        give |final keyframe| a |keyframe offset| of one (''100%'')
-        and append it to |keyframes|.
-
-    1.  Add to the |keyframe properties| of |initial keyframe| and |final keyframe|
-        the [=computed value=] on |element|
-        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.  For any property in |animated properties|
+        that is not otherwise present in a keyframe with an offset of ''0%''
+        or one that would be positioned earlier in the [=used keyframe order=],
+        add the [=computed value=] of that property on |element|
+        to |initial keyframe|’s |keyframe values|.
+
+    1.  If |initial keyframe|’s |keyframe values| is not empty,
+        prepend |initial keyframe| to |keyframes|.
+
+    1.  Repeat for |final keyframe|,
+        using an offset of ''100%'',
+        considering keyframes positioned <em>later</em> in the [=used keyframe order=],
+        and <em>appending</em> to |keyframes|.
 
 1.  <strong>Sort Frames:</strong>