The current the format for video supported by browsers now in use would be theora.ogv; it is supported by firefox and chrome, as chrome uses web kit it may work with safari, it does not work with Internet Explorer.
The easiest software to use to change the format into theora.ogv is http://www.mirovideoconverter.com/
WebM (vp8) looks like it will end being the standard in the end but the current releases of Firefox, Chrome, Safari and Internet Explorer to not yet support it.
The mark up is
<video controls width=”400″ height=”300″>
<source src=”video-file-theora.ogv” >
Update: add “AddType video/ogg .ogv” to .htaccess in the directory that contains the video if .ogv is not a configured mime type for your web server.
Safari 4 needs AddType video/mp4 mp4
<source src=”video-file.mp4″ >