Because the codecs are unsandboxed native code running in the browser process with full access to your user data. That you downloaded from the internet. To enjo

Because the codecs are unsandboxed native code running in the browser process with full access to your user data. That you downloaded from the internet. To enjoy pornography and pirated media (and pirated pornography!). It'd be a perfect exploit delivery vehicle, and on a newly-prominent attack surface (see the recent @font-face exploits) It's a much bigger problem on Windows because shitty DirectShow codec packs have a huge install base -- I wouldn't be surprised at a hundred million! It'll be a little better now since XP users won't get it, and the codec pack situation has cleaned up a lot in the last few years. The IE team is doing the right thing for the situation they have. Anybody that cares is going to be using a different browser, their support for <video> just means that we won't have to use a Flash fallback for Vista/W7 users that allow updates. Google is unlikely to prompt IE users to install the WebM codec on Youtube, since it'll work better with h.264 anyway. Apple doesn't use a whitelist now, but they might in the future. Their attack surface is more constrained, since it's only the code they ship directly (the library, their default codecs, and the pro stuff they ship with Final Cut), Perian/libavcodec, and Xiph/liboggplay that are actually installed anywhere. No cambrian explosion of repackaged hacked codecs to deal with.