[css-cascade] Add to the Filtering Declarations step that a declaration must be syntactically valid.

--HG--
extra : rebase_source : f99cc001b1e04a8894bfa310d0b21f1b000f0a6c

Author: tabatkins

css-cascade/Overview.html

@@ -504,6 +504,10 @@ <h3 id=filter-declarations><span class=secno>4.1. </span> Filtering
     href="#SELECT" rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a> (Taking <a
     href="http://www.w3.org/TR/selectors4/#scoping">scoping</a> into account,
     if necessary.)
+
+   <li> It is syntactically valid: the declaration's property is a known
+    property name, and the declaration's value matches the syntax for that
+    property.
   </ul>
 
   <p> The declarations that apply form, for each property on each element, a

css-cascade/Overview.src.html

@@ -299,6 +299,11 @@ <h3 id='filter-declarations'>
 		<li>
 			It belongs to a style rule whose selector matches the element. [[!SELECT]]
 			(Taking <a href="http://www.w3.org/TR/selectors4/#scoping">scoping</a> into account, if necessary.)
+
+		<li>
+			It is syntactically valid:
+			the declaration's property is a known property name,
+			and the declaration's value matches the syntax for that property.
 	</ul>
 
 	<p>