Add spec for appearance:base

This was discussed here: w3c#5998

css-ui-4/Overview.bs

@@ -2111,7 +2111,7 @@ Switching appearance: the 'appearance' property</h3>
 
 	<pre class="propdef">
 	Name: appearance
-	Value: ''appearance/none'' | ''auto'' | <<compat-auto>> | <<compat-special>>
+	Value: ''appearance/none'' | ''auto'' | ''base'' | <<compat-auto>> | <<compat-special>>
 	Initial: none
 	Applies To: all elements
 	Inherited: no
@@ -2133,7 +2133,10 @@ Switching appearance: the 'appearance' property</h3>
 	In particular, using ''appearance: none'' allows authors
 	to suppress the <a>native appearance</a> of <a>widgets</a>,
 	giving them a <dfn export>primitive appearance</dfn>
-	where CSS can be used to restyle them.
+  where CSS can be used to restyle them. Using ''appearance: base'' also
+  suppresses the <a>native appearance</a> of <a>widgets</a> and additionally
+  enables additional styling capabilities and interoperability, giving them a
+  <dfn export>base appearance</dfn>.
 
 	<details class=note>
 		<summary>Note on the history of this feature</summary>
@@ -2199,6 +2202,18 @@ Switching appearance: the 'appearance' property</h3>
 
 			Elements other than <a>widgets</a> must be rendered as for ''appearance/none''.
 
+    <dt><dfn>base</dfn>
+    <dd>
+      Just like ''appearance/none'', the element is rendered following the
+      usual rules of CSS. Replaced elements other than <a>widgets</a> are not
+      affected by this and remain replaced elements. <a>Widgets</a> must not
+      have their <a>native appearance</a>, and instead must have their <a>base
+      appearance</a>.
+
+      <a>Widgets</a> which don't have a <a>base appearance</a> must be rendered
+      as for ''appearance/auto''. The host language is responsible for defining
+      which <a>widgets</a> have a <a>base appearance</a>.
+
 		 <dt><dfn type="">&lt;compat-auto></dfn>
 		 <dd>
 			These values exist for compatibility of content developed