[css-overflow-5] Define ::scroll-button(*) to select all at once. #10722

Author: tabatkins

css-overflow-5/Overview.bs

@@ -416,7 +416,7 @@ Selecting The Active Scroll Marker: the '':target-current'' pseudo-class</h4>
 <h3 id="scroll-buttons">
 Scroll Buttons</h3>
 
-The <dfn>::scroll-button( <<scroll-button-direction>> )</dfn> pseudo-elements
+The <dfn>::scroll-button( '*' | <<scroll-button-direction>> )</dfn> pseudo-elements
 are generated on [=scroll containers=]
 when their computed 'content' value is not ''content/none''.
 They generate boxes as if they were immediately preceding <em>siblings</em>
@@ -451,7 +451,9 @@ or the verticals "around" the horizontals
 
 The four ''::scroll-button()'' pseudo-elements are individually selected 
 by the selector's argument.
-The possible <dfn><<scroll-button-direction>></dfn> values are:
+A '*' arguments selects all four ''::scroll-button()''s;
+otherwise the selected pseudo-element is determined 
+by the <dfn><<scroll-button-direction>></dfn> value:
 
 <dl dfn-type=value dfn-for="<scroll-button-direction>, ::scroll-button()">
 	: <dfn>up</dfn>