When you look at HTML5 video support of the five most popular web browsers, you notice that there is not a single browser supporting the two formats WebM and H.264. Depending on your browser choice, you end up either with one or the other, but not both.
Problems will arise for Internet users once the acceptance and use of HTML5 video rises on the Internet. The clear winner for now is Adobe with its Flash technology, as it is being used as a fallback on many sites. A common misconception surrounds H.264. Many users believe that it might replace Flash on the Internet. That's however not possible considering that H.264 is a video codec and Flash Player a multimedia runtime. H.264 encoded videos need to be loaded into a player, like Flash on the web or a desktop video player.
Regardless of that, it is still true that Flash is used as a fallback if the web browser does not support the HTML5 video format that is embedded on a page. To paraphrase: Flash based players are the most common to play H.264 contents, and Flash may be used as a fallback if the browser does not support the format or WebM.
Here is how you can test if your web browser supports WebM or H.264. Visit the following links with your browser of choice. Your browser supports the standard if the videos play, if you get errors, the standard is not supported.
WebM Test Video
Now that you found out that your browser is not capable of playing at least one of the formats, you may want to add support for the missing format to play all HTML5 videos regardless of codec.
Microsoft recently has published extensions for the Mozilla Firefox and Google Chrome web browser that add H.264 support to the browser. It has to be noted that the extension will only work under Windows 7, previous operating systems are not compatible.
Google on the other hand has created a plugin for Internet Explorer 9 that adds WebM to the web browser.
WebM Video for Microsoft Internet Explorer 9 (Preview) [download]
Instead of supporting both codecs in their web browser, Microsoft and Google have made the decision to produce plugins for each other's browser to add support for the video codec that they favor and support.
Microsoft made an announcement a while ago that Internet Explorer 9 would support the VP8 codec on Windows 7 if it was installed on the user system.
Please let us know in the comments if you have successfully downloaded the VP8 codec and managed to play WebM videos in Internet Explorer 9 afterwards.