Per bug 124128 comment 13, phrasing content (http://www.w3.org/TR/html5/dom.html#phrasing-content-1) should be accepted in <mo> elements.
Note however that the RelaxNG schema of the HTML5 validator currently only allows foreign content in <mtext> so that's what people are going to use in practice.
Created attachment 282220 [details]
So this one is very easy to fix now that RenderMathMLOperator stopped creating & destroying anonymous block & text nodes.
Comment on attachment 282220 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=282220&action=review
> + After r202420, the RenderMathMLOperator element no longer messes up with anonymous block and
"no longer messes with anonymous block and"
Committed r202572: <http://trac.webkit.org/changeset/202572>