To find WebM videos on YouTube:
- Search using the YouTube search
- On the URL for the search and "&webm=1&html5=1" and visit that URL.
- Click on a video.
- Add to the end of that video URL: &html5=1
- Enjoy WebM goodness.
- Right click in Firefox on the video to get fullscreen.
This post is available in the Ukranian language.