[web-animation-2] Include CSSNumberish as a valid type for EffectTiming duration (w3c#6703)

getComputedTiming resolves the iteration duration if specified as 'auto'. The resolved duration is CSSNumberish when the associated animation has a progress based timeline.

Author: kevers-google

web-animations-2/Overview.bs

@@ -2262,7 +2262,9 @@ partial interface AnimationEffect {
     needs to indicate that if <code>timing.duration</code>
     is the string <code>auto</code>, this attribute will return
     the current calculated value of the
-    <a>intrinsic iteration duration</a>.
+    <a>intrinsic iteration duration</a>, which may be a expressed as a double
+    representing the duration in milliseconds or a percentage when the effect
+    is associated with a [=progress based timeline=].
 
 :   <dfn attribute for=AnimationEffect>parent</dfn>
 ::  The <a>parent group</a> of this <a>animation effect</a> or
@@ -2349,13 +2351,20 @@ simply as removing the animation from its animation?
 <pre class="idl">
 partial dictionary EffectTiming {
     double playbackRate = 1.0;
+    (unrestricted double or CSSNumericValue or DOMString) duration = "auto";
 };
 
 partial dictionary OptionalEffectTiming {
     double playbackRate;
 };
 </pre>
 
+Note: In this version of the spec, duration is not settable as a
+CSSNumericValue; however, duration may be returned as a CSSNumericValue when
+resolving the duration in {{AnimationEffect/getComputedTiming()}}. Future
+versions of the spec may enable setting the duration as a CSSNumeric value,
+where the unit is a valid time unit or percent.
+
 <div class="attributes">
 
 :   <dfn>delay</dfn>
@@ -2383,11 +2392,11 @@ partial dictionary OptionalEffectTiming {
 
     > The <dfn>specified iteration duration</dfn> which is a real number greater
     > than or equal to zero (including positive infinity) representing the
-    > time taken to complete a single iteration of the <a>animation
-    > effect</a>, or the string value <code>auto</code> to indicate that the
-    > <a>iteration duration</a> reflects the animation effect's
-    > <a>intrinsic iteration duration</a>.  The |specified iteration duration|
-    > is converted to an <a>iteration duration</a> following  the
+    > time taken in milliseconds to complete a single iteration of the
+    > <a>animation effect</a>, or the string value <code>auto</code> to indicate
+    > that the <a>iteration duration</a> reflects the animation effect's
+    > <a>intrinsic iteration duration</a>. The |specified iteration duration| is
+    > converted to an <a>iteration duration</a> following the
     > [=normalize specified timing=] procedure.
 
 :   <dfn dict-member for=EffectTiming>playbackRate</dfn>