>>29489686
>False, and mostly vague.
It's not false. You don't have to take my word for it either. Just take any .mp4 video off of here, find out what the bitrate of it is, and then re-encode it as a VP9 .webm max quality settings at the exact same bitrate and you will end up with a smaller file. So if you want to optimize to the max and get the most bang for your bitrate, .webm is objectively the best way to go.
That being said, things could be even better still if this site would allow support for the AV1 codec which has a 30% improvement on compression over VP9. But out of what this site does support, VP9 is objectively the best format.
.mp4 might have more compatibility on other websites, but if you care about that maybe you should just be on those other websites rather than here.