HTML requires the <IMG> tag; it doesn't dictate requied media types.
HTML 5 defines the <VIDEO> tag, but doesn't dictate required codecs. Now.
Sure, it would have been nice, but there's nothing close to a compatible codec everyone could agree on - H.264's licensing is an issue for some, and Theora's lack of hardware support/acceleration is equally problematic to others. Cè la vie.
Now where's the Videoblock plugin for Firefox? :-)