Tracking the implementation of MathML in the wild
|Firefox||Currently the most feature-rich browser implementation of MathML.|
|Internet Explorer||No support in core distribution but with (no cost) MathPlayer add-on has full Content and Presentation MathML3 rendering (with non standard DOM due to IE's plugin architecture)|
|Opera||Only interested in implementing MathML for CSS.|
|Safari||Basic Support for a subset of Presentation MathML|
Content MathML should not be used and it is not going to be implemented by browser vendors. Any implementation of MathML should implement Presentation MathML. Firefox made sure that the Gecko engine does not corrupt copy-paste operations that contain Content MathML alternatives.
Mediawiki and Wikipedia
Right now the Mediawiki site has MathJax in testing and this will come to Wikipedia in the near future, when the new code gets deployed.