This is an example file for an HTML5 Video tutorial: http://edutechwiki.unige.ch/en/HTML5_audio_and_video
Look at the source if you don't want to read the EduTechWiki article ...
In this section we first will display different video formats. That way you can test your browser. We finally show how to define alternatives.
This example wouldn't work in a browser like IE9 that doesn't support webm
W3C validator