Add MathMLIDL

closes #5513 as per resolution from the minutes there documenting reality of what is shipping.

Author: bkardell

cssom-1/Overview.bs

@@ -2819,6 +2819,11 @@ If the user agent supports SVG, the following IDL applies: [[SVG11]]
 SVGElement includes ElementCSSInlineStyle;
 </pre>
 
+If the user agent supports MathML, the following IDL applies: [[MathML-Core]]
+
+<pre class=idl>
+MathMLElement includes ElementCSSInlineStyle;
+</pre>
 
 Extensions to the {{Window}} Interface {#extensions-to-the-window-interface}
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