WebM is a dead format. Although it purports to be patent free, it has very little support and may have exactly the same problems as H.264 with respect to patents.
It earns a "nice try" from Google, but not a passing grade. It's no PNG.
You might find the lengthy discussion in the linked article answers your question.
To summarize:
1. Millions of visitors can't view WebM
2. Most of the video created today is H.264
thanks for the tl;dr summary, I may give the full discussion a try.