Thanks for letting me know. Unfortunately, I suspect that the website you were viewing was either transparently decoding the webm in JS or transcoding it on the server; I can’t find any info suggesting MobileSafari supports WebM in any version, and I just updated my iPhone but I still get a negative result over here:

https://www.webmproject.org/detect/

If you get a positive result, please let me know. I would be willing to jump to a beta for that…