* Well, not really *anything* imaginable, but a lot of them QuickTime and Windows Media are pluggable multimedia frameworks. And since Safari and IE9 use them for <video> support, any codec or container format supported by the respective frameworks, works in the browser. For QuickTime, the list for videos goes along the lines of 3GP, Apple Video, AVI, DV, Cinepak, H.261, H.262, H.263, H.264, Micro