Hi Alberto,
the <video> tag is part of the (not yet finalized) HTML 5 specification.
Some browsers have implemented it, but not all, not it's very risky to be using it at this point.
One of the sticking issues with regard to this tag is the codec that the video has to be in. As of writing this there is no codec that all the browser manufacturers can agree on. the last time I checked they'd agreed not to agree on a codec which means using this tag will be a complete balls-up down the road. Each video will need to be encoded in at least 2 different ways, and the server will need to determine which to supply.
cheers
Bruce