Added <element()> to <image> data type (w3c#5171)

Author: SebastianZ

css-images-4/Overview.bs

@@ -70,7 +70,7 @@ Value Definitions {#values}
 	or <a section href="#gradients">gradient notation</a>.
 	Its syntax is:
 
-	<pre class="prod"><dfn>&lt;image></dfn> = <<url>> | <<image()>> | <<image-set()>> | <<cross-fade()>> | <<gradient>></pre>
+	<pre class="prod"><dfn>&lt;image></dfn> = <<url>> | <<image()>> | <<image-set()>> | <<cross-fade()>> | <<element()>> | <<gradient>></pre>
 
 	An <<image>> can be used in many CSS properties,
 	including the 'background-image', 'list-style-image', 'cursor' properties [[!CSS21]]