[css-ui] Specify where appearance: button should work

Author: zcorpan

css-ui-4/Overview.bs

@@ -1843,13 +1843,10 @@ Animation type: discrete
 		The element is rendered with the look and feel of a push button,
 		similar to the ''appearance: auto'' rendering of the [[HTML]] <{button}> element.
 
-		UAs may treat this value as ''appearance/auto'' on form controls
-		or replaced elements
-		for which it is impractical or nonsensical.
+		UAs must treat this value as ''appearance/auto'' on <{input}> elements,
+		<{textarea}> elements, <{meter}> elements, and <{progress}> elements.
 
-		Issue: Are there some elements
-		on which this value <em>must</em> be treated as auto for web compat requirements?
-		The phrasing above alows it, and that's probably enough.
+		Note: Using ''appearance/button'' on a <{select}> element works.
 
 	<dt><dfn type="">&lt;compat></dfn>
 	<dd>