[web-animations-2] Expand offset syntax for keyframes w3c#7637

Author: fantasai

web-animations-2/Overview.bs

@@ -2937,6 +2937,27 @@ new SequenceEffect(
 
 </div>
 
+<h4 id="keyframe-offset-type">Modification to the <code>*Keyframe</code> dictionaries</h4>
+
+The <code>double?</code> type of the <code>offset</code> key
+in <code>ComputedKeyframe</code>,
+<code>BaseComputedKeyframe</code>,
+<code>BaseKeyframe</code>,
+<code>BasePropertyIndexedKeyframe</code>,
+etc.
+is replaced by <code>(CSSNumberish? or TimelineRangeOffset or DOMString)</code>,
+where <code>double</code> continues to be interpreted as a percentage,
+a {{DOMString}} is parsed into a {{TimelineRangeOffset}}
+according to the syntax of <<keyframe-selector>>,
+and the resulting {{TimelineRangeOffset}} or {{CSSNumericValue}}
+is interpreted as defined for ''@keyframes''.
+
+If the {{DOMString}} fails to parse as a valid <<keyframe-selector>>,
+then the keyframe is invalid,
+and will throw a <span class=exceptionname>TypeError</span>
+when [[web-animations-1#processing-a-keyframes-argument|processed]]
+(same as if the <code>doube</code> value was outside the [0,1] range).
+
 <h4 id="the-keyframeeffectoptions-dictionary">The KeyframeEffectOptions dictionary</h4>
 
 The {{KeyframeEffectOptions}} dictionary interface is modified to add the