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It's pre-installed with Windows since W95, and also comes with IE (even on Mac) and with MS Office (even on Mac), so that must account for well over 98% of users. Webdings (and Wing2) are probably close behind. And of course, it's free, so Linux and Amiga users may have it too. Anyway, that's explicitly what those fonts are designed for.
As for browser support, I just tested and it works with IE5, IE6, Netscape 4, Netscape 7. Mozilla Phoenix and Firefox, OffByOne, but not Opera 5...