More clarifiction of how attributes are parsed.

Author: tabatkins

css3-values/Overview.html

@@ -1673,6 +1673,15 @@ <h3 id=attr-notation><span class=secno>8.3. </span> Attribute References:
     &lsquo;<code class=css>inset</code>&rsquo; keyword.
   </div>
 
+  <p> If the specified attribute exists on the element, the value of the
+   attribute must be trimmed of leading and trailing white space and then
+   parsed against the <code>any</code> production in the Core Grammar defined
+   by CSS 2.1 <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
+   If it is successfully parsed as required by the
+   <var>&lt;type-or-unit></var> argument (as defined in the list below), the
+   resulting value is the attr() expression's value. Otherwise, the attr()
+   expression's value is its fallback value.
+
   <p>The <var>&lt;type-or-unit></var> keywords are:
 
   <dl>

css3-values/Overview.src.html

@@ -1191,7 +1191,15 @@ <h3 id="attr-notation">
 			with either a ''px'' length <em>or</em> the ''inset'' keyword.
 	</div>
 
-
+	<p>
+		If the specified attribute exists on the element, 
+		the value of the attribute must be trimmed of leading and trailing white space
+		and then parsed against the <code>any</code> production
+		in the Core Grammar defined by CSS 2.1 [[!CSS21]]
+		If it is successfully parsed as required by the <var>&lt;type-or-unit></var> argument
+		(as defined in the list below),
+		the resulting value is the attr() expression's value.
+		Otherwise, the attr() expression's value is its fallback value.
 
 	<p>The <var>&lt;type-or-unit></var> keywords are:</p>