If ffmpeg was a JS library, it wouldn't need to have anything to do with the browser vendors, it would be provided by the content host. The only concern is the

If ffmpeg was a JS library, it wouldn't need to have anything to do with the browser vendors, it would be provided by the content host. The only concern is the binary size (and hey, that could be optimized by dropping codecs from the build that don't appear on the site).