This means that folks won’t have to download a whole bunch of different codecs or plugins when using different browsers to watch videos on YouTube. This should also mean that mobile devices like smartphones can access the same content as desktop computers so there’s no more need to create mobile-optimized content.
YouTube has already started the transcoding process, converting all newly uploaded videos into WebM. At the moment only 30% of videos have been converted, which might not seem like a lot, but apparently it makes up for 99% of the videos we watch today. When the servers aren’t so busy, they will be converting all the old videos to this new format as well.
YouTube will still continue to support H.264 so users of browsers and devices that have no problem playing YouTube videos now won’t have to worry, but who knows for how long?