IE 8 doesn't support mp4 or ogv. Chrome supports both. Firefox supports ogv and webm. Safari supports mp4 and mov.
[Premiere doesn't support export to .ogv or .webm format. VLC can be used to covert a video to .ogv or .webm format. To save a video to one of these formats, open VLC, select Media / Convert(save). Click Add to add a video. Select Convert/Save. Specify destination and select Profile type.]
Note, if you leave off the type attribute on the source tag, the browser will have to download the video to decide if it can or can't play it. Actually the browser needs to also known the codec which is the codecs tag (http://my.opera.com/core/blog/2010/03/03/everything-you-need-to-know-about-html5-video-and-audio-2).
Note, the web server needs to return the correct mime type for .mp4 and .ogv files. If the web server is IIS you can add a Web.config file if necessary. See the Web.config file in the current dir for an example.
mov:
mp4:
mp4(mk):
ogv:
webm:
Multiple values for source: