[css-display] Define that display:contents behaves as display:none for all MathML elements. Fixes w3c#2167.

Author: tabatkins

css-display-3/Overview.bs

@@ -1188,6 +1188,12 @@ SVG Elements {#unbox-svg}
 	where their contents are handled in a totally separate context from their parent,
 	just removes itself entirely.
 
+MathML Elements {#unbox-mathml}
+-------------------------------
+
+	For all MathML elements,
+	''display: contents'' behaves as ''display: none''.
+
 <h2 class=no-num id="box-guidelines">
 Appendix C: Box Construction Guidelines for Spec Authors</h2>