[css-values-4] Clarify guidance on categorization of interface changes wrt viewport units. w3c#4329

Author: fantasai

css-values-4/Overview.bs

@@ -1872,6 +1872,10 @@ The Large, Small, and Dynamic Viewport Sizes</h5>
 	any changes in interface that happen as a result of scrolling
 	or other frequent page interactions that would disturb the user
 	if they resulted in substantial layout changes.
+	UAs <em>should</em> categorize as the former
+	any changes in interface that have a sufficiently steady state
+	that re-laying out the document into the adjusted space
+	would be beneficial to the user.
 	Additionally,
 	UAs may have some dynamically-shown interfaces which intentionally overlay content
 	and do not cause any shifts in layout