[css-image-animation] Define what state 'running' starts from (#13638)

This is in response to #13429.

Author: frivoal

css-image-animation-1/Overview.bs

@@ -151,7 +151,7 @@ Controlling Image Animations: the 'image-animation' property {#image-animation}
 		<dd>
 			Like ''normal'',
 			the animation of [=animated images=] is run normally,
-			as determined by the image format and the host language,
+			as determined by the image format and the host language.
 
 			However,
 			animation timelines are scoped per element:
@@ -160,8 +160,19 @@ Controlling Image Animations: the 'image-animation' property {#image-animation}
 			the same image data,
 			and with an 'image-animation' computed value of ''running''
 			must be rendered synchronized to the same timeline as a group,
-			distinct from the timeline of images in other elements,
-			with the timeline starting at the time of the least recent addition to the group.
+			distinct from the timeline of images in other elements.
+
+			If images are added to the element while the computed value is ''running'',
+			the timeline starts at the time of the least recent addition to the group.
+
+			If the element is created or made visible after having previously been set to ''display: none'',
+			with images already added and with 'image-animation' already set to ''running'',
+			the timeline starts when the element is included in the layout.
+
+			If this property is switched to ''running'' from another value,
+			the beginnig of this timeline is set
+			so that the animation continues from the state that was displayed
+			at the timem of the switch.
 	</dl>
 
 	<div class=example id=prefers>