[css-ui-4] define appearance: base-select (#12402)

This was resolved here:
#10440 (comment)

Base appearance for the select element and its picker is going to be
published in the HTML spec very soon.

Author: josepharhar

css-ui-4/Overview.bs

@@ -2235,7 +2235,7 @@ Switching appearance: the 'appearance' property</h3>
 
 	<pre class="propdef">
 	Name: appearance
-	Value: ''appearance/none'' | ''auto'' | ''base'' | <<compat-auto>> | <<compat-special>>
+	Value: ''appearance/none'' | ''auto'' | ''base'' | ''base-select'' | <<compat-auto>> | <<compat-special>>
 	Initial: none
 	Applies To: all elements
 	Inherited: no
@@ -2358,6 +2358,17 @@ Switching appearance: the 'appearance' property</h3>
 					for defining which <a>widgets</a> have a <a>base appearance</a>.
 			</dl>
 
+		<dt><dfn>base-select</dfn>
+			<dd>
+				The effect of ''appearance/base-select'' depends on the element it is applied to:
+				<dl class=switch>
+					: The <{select}> element or ''::picker(select)''
+					:: The element is rendered as for ''appearance/base''.
+
+					: Everything else
+					:: The element is rendered as for ''appearance/auto''.
+				</dl>
+
 		 <dt><dfn type="">&lt;compat-auto></dfn>
 		 <dd>
 			These values exist for compatibility of content developed