[css-image-animation] Treat video poster image as image

Closes #13641

Author: frivoal

css-image-animation-1/Overview.bs

@@ -343,6 +343,10 @@ Terminology {#terminology}
 			or the <{image}> [[SVG2]] element
 			represent [=content images=].
 
+			When it represents its <a spec=html>poster frame</a> (and only then),
+			the HTML <{video}> element
+			is considered a [=content image=].
+
 		<dt><dfn>decorative image</dfn>
 		<dd>
 			An image inserted into the document rendering by CSS,
@@ -391,12 +395,6 @@ Terminology {#terminology}
 			distinctly from the initial state of the animation.
 	</dl>
 
-	Issue(w3c/csswg-drafts#13641): Should the <{video}> element
-	when it represents its <a spec=html>poster frame</a>
-	be treated as a [=content image=],
-	a [=decorative image=],
-	or as (a state of) a [=video=] and not an image at all?
-
 
 Accessibility Considerations {#a11y}
 ====================================