[css-backgrounds-4] Add background-clip: border

Author: LeaVerou

css-backgrounds-4/Overview.bs

@@ -132,7 +132,7 @@ Painting Area: the 'background-clip' property</h3>
 
 	<pre class="propdef partial">
 	Name: background-clip
-	New Values: text
+	New Values: text, border
 	</pre>
 
 	Issue: Or should this be defining the <css>-webkit-background-clip</css> property,
@@ -143,8 +143,13 @@ Painting Area: the 'background-clip' property</h3>
 		<dt><dfn>text</dfn>
 		<dd>
 			The background is painted within (clipped to)
-			the union of the border box
+			the intersection of the border box
 			and the geometry of the text in the element and its in-flow and floated descendants.
+
+		<dt><dfn>border</dfn></dt>
+		<dd>
+			The background is clipped to the area painted by the border, taking 'border-width' and 'border-style' into account but ignoring any transparency introduced by 'border-color'.
+		</dd>
 	</dl>
 
 <h2 id="borders">
@@ -692,7 +697,7 @@ Additions Since Level 3</h3>
 		<li>logical 'background-position' values (''background-position/start'', ''background-position/end'')
 		<li>the ''extend'' keyword of 'background-repeat'
 		<li>'corner-shape'
-		<li>multiple borders per element
+		<li>multiple border colors per border
 		<li>logical border properties
 		<li><a href="#partial-borders">Partial Borders</a> (make part of border shorthand as well!)
 		<li><a href="http://lists.w3.org/Archives/Public/www-style/2012Oct/0314.html">More <css>border-radius</css> shorthands</a> for doing both corners on a side at once.