[css-content-3] content: normal needs to compute to normal on ::marker

Author: fantasai

css-content-3/Overview.bs

@@ -118,6 +118,8 @@ Inserting and replacing content with the 'content' property</h2>
 
 			For ''::before'' and ''::after'', this computes to ''content/none''.
 
+			For ''::marker'', this computes to itself (''content/normal'').
+
 		<dt><dfn>none</dfn>
 		<dd>
 			On elements, this inhibits the children of the element from being rendered as children of this element,