My current browser, WebKit r64727, has native MathML support.
On your MathML examples page, /demos/mathml-samples/, you display the text "Note: Your browser does not support Native MathML rendering" which is quite incorrect, and then you replace all the MathML (which looked sub-optimal) with HTML spans (which looked good).
Disabling javascript and reloading the page fixes this "problem".
With JS (left) vs. WIthout JS (right) in Safari.
Yes, MathJax doesn't actually "detect" native MathML support, it uses a table to know implementations. It would be nice to be able to detect it, and I have some ideas for that, but that's not the current behavior. I'll put it on the "to do" list for the future.
Davide