I don't believe that MathML should be integrated in a browser: MathML does not requires a lot of CPU, so you can get away with JS+canvas that implements the MathML spec.

By having it in the browser you are making the browser more bloated (more disk space, more memory, more cache misses) and possibly less stable/secure, even-though less than 1% of the people are going to use this feature. Other issues include increasing the barrier of entry for new browsers, having to deal with differences in implementation of MathML when using it, and making the spec harder to improve.