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MathML font fallback
Firefox has the following core functionality (i.e. not an extension) built in as part of its MathML engine:font.mathfont-family is a user preferences key that takes in a string of the form:
Font 1,Font 2,Font 3,...,Font n
The order of this list determines the MathML font priority in the following way: When Firefox renders MathML, it attempts to grab as many glyphs as possible from Font 1, then falls back to Font 2 and tries to load as many glyphs as possible, then falls back to Font 3,... and so on (obviously for n fonts where n is a natural number).
At the moment, Opera can only set a single MathML font (and only by messing directly with the MathML CSS file), but if it is missing glyphs, it does not fall back and just displays an empty box.
The STIX project (supported by the AIP, ACS, AMS, IEEE, and APS) is releasing ver 1.0 of its technical fonts for MathML either today or tomorrow, and this doesn't seem too hard to hack together. I'm sure that you could publish a very nice press release saying something like "Opera's MathML support has improved".
I'm a longtime fan of Opera, but I'm also a mathematician, and it's rather unfortunate that Opera has totally slacked on MathML support. There are similar sentiments in the physics and chemistry communities (c.f. Jacques Distler's blog, for instance). I hope the Opera overlords will find the time to improve MathML support.