Would you mind giving us the source of the video?
<source src="source of the video" type="video/mp4"/>
???
--- Edit ---
When I had trouble playing a video in HTML, I used the iframe tag instead of the video tag:
Here's a link to what I have.
http://codepad.org/umgTY0bF
I validated the HTML so I had to add a DOCTYPE (this specifies HTML5) and I added a <title> element since it is required to validate, and I added a declaration of UTF-8 as the charset. It's good to have pages that validate.
