[css-containment] Add some optimization use-cases.

Author: tabatkins

css-containment/Overview.bs

@@ -122,6 +122,8 @@ Types of Containment</h2>
 	and helps authors compose their page out of functional units,
 	as it limits how widely an effect a given change can have on a document.
 
+	Issue: What other effects need to be built into the containment types?
+
 <h3 id='containment-layout'>
 Layout Containment</h3>
 
@@ -137,7 +139,18 @@ Layout Containment</h3>
 
 	Possible optimizations that can be enabled by <a>layout containment</a> include (but are not limited to):
 
-	...
+	1. When the style or contents of a descendant of the containing element is changed,
+		calculating what part of the DOM tree is "dirtied" and might need to be re-laid out
+		can stop at the containing element.
+
+	2. When laying out the page,
+		if the containing element is off-screen or obscured,
+		the layout of its contents can be delayed or done at a lower priority.
+
+	3. When laying out the page,
+		the contents of separate containing elements
+		can be laid out in parallel,
+		as they're guaranteed not to affect each other.
 
 <h3 id='containment-style'>
 Style Containment</h3>
@@ -168,7 +181,9 @@ Style Containment</h3>
 
 	Possible optimizations that can be enabled by <a>style containment</a> include (but are not limited to):
 
-	...
+	1. Whenever a property is changed on a descendant of the containing element,
+		calculating what part of the DOM tree is "dirtied" and might need to have its style recalculated
+		can stop at the containing element.
 
 <h3 id='containment-paint'>
 Paint Containment</h3>
@@ -181,5 +196,11 @@ Paint Containment</h3>
 
 	Possible optimizations that can be enabled by <a>paint containment</a> include (but are not limited to):
 
-	...
+	1. If the containing element is off-screen or obscured,
+		the UA can directly skip trying to paint its contents,
+		as they're guaranteed to be off-screen/obscured as well.
 
+	2. If the containing element is ''overflow:visible'',
+		the UA can reserve "canvas" space for the element exactly the element's size.
+		(Normally, UAs often predictively overpaint elements somewhat,
+		so scrolls can start showing content immediately.)