MathML Fonts
Firefox, Mozilla, and Netscape 7.X support MathML rendering directly with no plugin required. They do, however, require that you have Math fonts installed. You can install the necessary fonts for Windows by clicking on this link. For Macintosh and Linux platforms, please follow the font installation instructions found here.
Internet Explorer 6.0 and 7.0 support MathML rendering with the Design Science Math Player plugin, which you can obtain by clicking on this link.
