[css-ui-4] Define appearance:base (#10691)

This was discussed here: #5998

Author: josepharhar

css-ui-4/Overview.bs

@@ -2116,7 +2116,7 @@ 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
@@ -2140,6 +2140,22 @@ Switching appearance: the 'appearance' property</h3>
 	giving them a <dfn export>primitive appearance</dfn>
 	where CSS can be used to restyle them.
 
+	Using ''appearance: base'' gives <a>widgets</a> a <dfn export>base
+	appearance</dfn>. <a>Widgets</a> in <span>base appearance</span> have their
+	<a>native appearance</a> suppressed where CSS can be used to restyle them,
+	just like <a>primitive appearance</a>, but also have default styles which are
+	usable and interoperable. This is unlike <a>primitive appearance</a> which
+	can cause some <a>widgets</a> to disappear completely.
+
+	A widget can have some style sheet associated with it and those styles can
+	change based on whether the widget has a <span>base appearance</span>.
+	<span>Native appearance</span> and <span>primitive appearance</span> should
+	have the same styles.
+
+	ISSUE: ''appearance:base'' is not ready to ship until its stylesheet has been
+	fully defined in the HTML specification, and implemented accordingly, for all
+	HTML form controls.
+
 	<details class=note>
 		<summary>Note on the history of this feature</summary>
 
@@ -2204,6 +2220,25 @@ Switching appearance: the 'appearance' property</h3>
 
 			Elements other than <a>widgets</a> must be rendered as for ''appearance/none''.
 
+		<dt><dfn>base</dfn>
+		<dd>
+			The effect of ''appearance/base'' depends on the element it is applied to:
+			<dl class=switch>
+				: elements other than <a>widgets</a>
+				:: Elements other than <a>widgets</a> must be rendered as for
+					''appearance/none''.
+
+				: <a>Widgets</a> which support <a>base appearance</a>
+				:: Just like ''appearance/none'', the element is rendered following the
+					usual rules of CSS. <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 support <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>.
+			</dl>
+
 		 <dt><dfn type="">&lt;compat-auto></dfn>
 		 <dd>
 			These values exist for compatibility of content developed