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It should be possible to select MathML, XHTML+MathML, XHTML+Math+SVG, etc. from the DCOTYPE popup.
Nuking this as I no longer recall why I thought it'd be a good idea to include these document types in the popup. Feel free to reopen if you do.
I actually added MathML, XHTML+MathML to the drop-down menu, upon request from the math staff contact, with the rationale that most MathML documents do not use a doctype, and thus having the override handy would make it easier to (tentatively) validate such docs.