LAS VEGAS--Microsoft won't say just when it expects to ship Internet Explorer 9, but whenever it does, it won't be running on Windows XP.

"Building a modern browser requires a modern operating system," Internet Explorer General Manager Dean Hachamovitch said at a press conference at the Mix10 event.

Microsoft unveiled a "platform preview" of IE9 earlier on Monday. That early version, which contains the Chakra engine, but not the full user interface (or even a back button) requires Vista Service Pack 2 or later.

One of the areas of focus was on the ability to play HTML 5 video. As for which video codecs Microsoft will support, Hachamovitch would only commit to H.264.