[css-image-animation] Distinguish between the paused and stopped states (#13636)

Addresses #13434

Author: frivoal

css-image-animation-1/Overview.bs

@@ -79,7 +79,7 @@ Controlling Image Animations: the 'image-animation' property {#image-animation}
 
 	<pre class="propdef">
 	Name: image-animation
-	Value: normal | paused | running
+	Value: normal | paused | stopped | running
 	Initial: normal
 	Applies to: [=content images=] and elements with [=decorative images=]
 	Inherited: yes
@@ -126,10 +126,26 @@ Controlling Image Animations: the 'image-animation' property {#image-animation}
 			and therefore does not normatively require the above behavior.
 			This specification does.
 
-		<dt><dfn>paused</dfn>
+		<dt><dfn>stopped</dfn>
 		<dd>
 			[=Animated images=] are rendered as if they were [=static images=]:
 			the user agent must not run any animation it contains.
+			If the image contains a [=cover frame=],
+			that is what the user agent must use;
+			otherwise, it must display the image
+			as it would be in the initial state of the animation.
+
+		<dt><dfn>paused</dfn>
+		<dd>
+			The user agent must not run the animation
+			contained in the [=Animated images=],
+			and must continue to display the image
+			as it was when this value started to apply,
+			effectively pausing the animation.
+
+			If the animation had not been playing
+			prior to this value being applied,
+			the behavior is the same as ''stopped''.
 
 		<dt><dfn>running</dfn>
 		<dd>
@@ -353,6 +369,15 @@ Terminology {#terminology}
 
 			<p class=example id=ex-p-i>
 			The <{canvas}> [[HTML]] element represents a [=programmatic image=].
+
+		<dt><dfn>cover frame</dfn>
+		<dd>
+			Some [=animated image=] formats make it possible
+			for the creator of the image
+			to specify a [=static image=],
+			referred to in this document as the [=cover frame=],
+			to display when the animation is not running,
+			distinctly from the initial state of the animation.
 	</dl>
 
 	Issue: Should the <{video}> element