Update the attr() URL type with a link to the about:invalid definition.

Author: tabatkins

css3-values/Overview.html

@@ -1752,11 +1752,11 @@ <h3 id=attr-notation><span class=secno>8.3. </span> Attribute References:
     href="#string-value"><var>&lt;string></var></a>. It is interpreted as a
     quoted string within the ‘<a href="#url"><code
     class=css>url()</code></a>’ notation. The default is ‘<code
-    class=css>about:invalid</code>’, which is a URI defined to point to a
-    non-existent document with a generic error condition. Relative URLs must
-    be made absolute according to the rules of the document language as
-    applied to URLs originating from the element; they are not relative to
-    the style sheet.
+    class=css>about:invalid</code>’, which is a URI defined (<a
+    href=about-invalid>in Appendix A</a>) to point to a non-existent document
+    with a generic error condition. Relative URLs must be made absolute
+    according to the rules of the document language as applied to URLs
+    originating from the element; they are not relative to the style sheet.
 
    <dt>‘<a href="#integer"><code class=css>integer</code></a>’
 

css3-values/Overview.src.html

@@ -1282,7 +1282,7 @@ <h3 id="attr-notation">
 		<dd>
 			The attribute value will be parsed as the contents of a CSS <var>&lt;string></var>. 
 			It is interpreted as a quoted string within the ''url()'' notation.
-			The default is ''about:invalid'', which is a URI defined to point
+			The default is ''about:invalid'', which is a URI defined (<a href="about-invalid">in Appendix A</a>) to point
 			to a non-existent document with a generic error condition. 
 			Relative URLs must be made absolute
 			according to the rules of the document language as applied to URLs originating from the element;