That is, if you limit yourself to one format only. If you provide Flash then it's probably H.264. In that case it's not 50% but 10% support for HTML5 Video. If you want to offer HTML5 Video for the 50% you need at least to transcode to Theora. And that, depending on scale, is not so trivial anymore.
Though that starts to look better if you consider good adoption of IE9 and Firefox on Windows with Microsoft's plugin.
Though that starts to look better if you consider good adoption of IE9 and Firefox on Windows with Microsoft's plugin.