If you are using Internet Explorer 9 or higher, and are unable to view WebM content (you get an "invalid source" error), download and install this software to fix it:
https://tools.google.com/dlpage/webmmf/
In the next update, an error bar will pop up under the video with this information when it fails to load, so this forum post is just to inform you guys about it before then.
Updated by MaxyMillion