[cssom] Include vendor-prefixed properties in "supported CSS property"

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=29440

Author: zcorpan

cssom/Overview.bs

@@ -159,9 +159,10 @@ The term <dfn export>supported styling language</dfn> refers to CSS.
 
 Note: If another styling language becomes supported in user agents, this specification is expected to be updated as necessary.
 
-The term <dfn export>supported CSS property</dfn> refers to a CSS property that the user agent implements,
-and that is defined to be a case-insensitive property in the CSS specification.
-A <a>supported CSS property</a> must be in its lowercase form for the purpose of comparisons this specification.
+The term <dfn export>supported CSS property</dfn> refers to a CSS property that the user agent
+implements, including any vendor-prefixed properties, but excluding <a>custom properties</a>. A
+<a>supported CSS property</a> must be in its lowercase form for the purpose of comparisons in this
+specification.
 
 In this specification the ''::before'' and ''::after'' pseudo-elements
 are assumed to exist for all elements even if no box is generated for them.