[css-ui] appearance: menulist-button is different from auto

Fixes #3551

Tests: web-platform-tests/wpt#14996

Author: zcorpan

css-ui-4/Overview.bs

@@ -86,6 +86,7 @@ spec:css-backgrounds-3; type:property; text:border-width
 spec:css-overflow-4; type:property; text:text-overflow
 spec:selectors-4; type:selector; text::enabled
 spec:selectors-4; type:selector; text::disabled
+spec:css-color-4; type:property; text:color
 </pre>
 
 <style>
@@ -1795,7 +1796,7 @@ so that CSS can be used to restyle them.
 
 <pre class="propdef">
 Name: appearance
-Value: ''appearance/none'' | ''auto'' | ''button'' | ''textfield'' | <<compat>>
+Value: ''appearance/none'' | ''auto'' | ''button'' | ''textfield'' | ''menulist-button'' | <<compat>>
 Initial: none
 Applies To: all elements
 Inherited: no
@@ -1856,13 +1857,23 @@ Animation type: discrete
 
 		For all other elements, this value has the same effect as ''appearance/auto''.
 
+	<dt><dfn>menulist-button</dfn>
+	<dd>
+		For <a spec="html">drop-down box</a> <{select}> elements,
+		the element is rendered as a drop-down box, including a "drop-down button",
+		but not necessarily using a native control of the host operating system.
+		For such elements, CSS properties such as 'color', 'background-color', and 'border'
+		(that can be disregarded for ''appearance/auto'') should not be disregarded.
+
+		For all other elements, this value has the same effect as ''appearance/auto''.
+
 	<dt><dfn type="">&lt;compat></dfn>
 	<dd>
 		These values exist for compatibility of content developed
 		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>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>menulist</dfn> | <dfn>menulist-button</dfn> | <dfn>listbox</dfn> | <dfn>meter</dfn> | <dfn>progress-bar</pre></dfn>
+		<pre class=prod style="white-space: normal"><<compat>> =  <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>menulist</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;