HTML5 video is essentially a standard that browsers can implement
to display video on a web page. Currently, the task of displaying video
generally falls to a Flash player. With Flash, Adobe has really pushed
the boundaries on displaying video on the web, but it can’t be good for
one vendor to control such an important delivery mechanism forever.
Essentially, HTML5 will mean that your browser will play video natively,
and you won’t have to visit Adobe to get Flash first. YouTube has for
some time offered an HTML5 version of its videos, and it’s likely this
approach will become commonplace as the standard matures.