[css-contain-2] Define how animations and transitions act on skipped elements. #5611

Author: tabatkins

css-contain-2/Overview.bs

@@ -1542,6 +1542,62 @@ Restrictions and Clarifications {#cv-notes}
 
 8. [=Skipped contents=] do not contribute to the result of {{HTMLElement/innerText}}.
 
+9. While an element is [=skipped=],
+	CSS transitions and animations on the element do not update:
+
+	* New animations are not created even if newly-applied style would start one.
+	* Existing animations do not advance in their timeline.
+	* Running animations on the element do not end.
+
+	If script queries the style of a [=skipped=] element
+	(causing a [=style change event=])
+	such that knowing the state of animations is required
+	to return correct information,
+	animations are "refreshed" at that moment
+	according to the styles at the time of that [=style change event=]:
+	new animations are created if necessary;
+	existing animations are advanced
+	to the point on their timeline they would have been at
+	were they never skipped at all;
+	animations that have would have ended since the last "refresh"
+	are ended.
+
+	If a transition or animation-related event would have been triggered
+	as a result of changes occurring during the "refresh" interval,
+	they are fired when the animations are "refreshed",
+	in the same order they would have originally fired in
+	had the element never been [=skipped=].
+	If multiple iterations of an animation would have completed during this period,
+	only one {{animationiteration}} event must be triggered,
+	ordered as if it were triggered at the first completed iteration during the refresh interval.
+
+	When an element stops being [=skipped=],
+	animations are refreshed
+	and then resume advancing on their timelines as normal from that point.
+
+	Note: Overall, this is similar to the behavior of transitions/animations
+	when a background tab is brought back to the foreground,
+	allowing user agents to skip as much unnecessary animation work as possible
+	without overly disrupting the animations when they become relevant again.
+
+10. While an element is [=skipped=],
+	it must not start any transitions,
+	even if a [=style change event=] affects its computed styles.
+
+	When an element stops being [=skipped=],
+	it must not start any transitions
+	as a result of the [=style change event=]
+	associated with it no longer being [=skipped=].
+
+	Note: This is similar to an element switching from ''display:none''
+	to a non-''display/none'' value--
+	even tho the styles are <em>technically</em> changing in that case
+	(from their initial values
+	to their "proper" values from the cascade),
+	no transitions are started.
+
+
+
 Accessibility Implications {#cv-a11y}
 -------------------------------------