[css-ui] Add button-bevel to the appearance <compat> list

Author: zcorpan

css-ui-4/Overview.bs

@@ -1857,7 +1857,7 @@ Animation type: discrete
 		for earlier non standard versions of this property.
 		They all have the same effect as ''appearance/auto''.
 
-		<pre class=prod style="white-space: normal"><<compat>> =  <dfn>textfield</dfn> | <dfn>searchfield</dfn> | <dfn>textarea</dfn> | <dfn>push-button</dfn> | <dfn>slider-horizontal</dfn> | <dfn>checkbox</dfn> | <dfn>radio</dfn> | <dfn>square-button</dfn> | <dfn>menu-list</dfn> | <dfn>listbox</dfn> | <dfn>meter</dfn> | <dfn>progress-bar</pre></dfn>
+		<pre class=prod style="white-space: normal"><<compat>> =  <dfn>textfield</dfn> | <dfn>searchfield</dfn> | <dfn>textarea</dfn> | <dfn>push-button</dfn> | <dfn>button-bevel</dfn> | <dfn>slider-horizontal</dfn> | <dfn>checkbox</dfn> | <dfn>radio</dfn> | <dfn>square-button</dfn> | <dfn>menu-list</dfn> | <dfn>listbox</dfn> | <dfn>meter</dfn> | <dfn>progress-bar</pre></dfn>
 
 		Issue: If any of these value is not needed for web compat,
 		they should be dropped from this list;