WebM Cons:
Might have to click to start
Some mobile browsers won't support them.
WebM Pros:
Everything else
Poster in not reading his own source shocker. Also Firefox doesn't support H264 by default either.WebM support: http://caniuse.com/webm
seems that only Firefox, Chrome and Opera support WebM.
Which means that on desktop, if wouldn't work for IE and Safari users either. Not exactly widespread support as yet (and probably not likely to expand since Microsoft and Apple want to support H.264 as the de-facto video format).
As previously mentioned by someone, might as well just have youtube embeds
Note: Will work in IE9+ and Safari/MacOSX provided the user has the WebM codecs installed. Partial support indicates that at least one codec is supported but not all.
Poster in not reading his own source shocker.
The Bible, bringing joy to millions..