MediaElement.js is a nifty jQuery plugin that allows you to use the HTML5 video and audio elements while still supplying a Flash or Silverlight backup for compatibility.
There’s now a convenient MediaElement.js WordPress plugin that gives you easy-to-use shortcodes for use in the post editor. To embed a video when you have the plugin installed, all you have to do is put this in your post:
[video src="http://example.org/video.mp4" width="640" height="480"]
The MPEG 4 video will be served-up to supporting browsers, and others get the Flash backup. Supported formats also include WebM, MP3 and Ogg Theora. Multiple formats can be specified in one shortcode, if you have encoded multiple versions. You can serve Safari and Mobile Safari an MP4 version, Firefox an Ogg Theora, and Chrome the little-supported WebM.