Better web video with AV1 codec

File extensions ( , , or  ) barely represent containers, up to three different formats are used to make a video file happen:

So, when you see extension, the only thing you can be sure about is that the MP4 container had been used to package a file. With all the low-level trickery involved, AV1 is capable of generating files that are up to 30-50% smaller than H.264/VP8 and up to 30% smaller than HEVC, even though, due to being still mostly experimental, it has some problems (at the time of this writing):

The central promise for of AV1 is maintaining high image quality even at low bitrates, thus allowing for smaller files without apparent compression artifacts. Type attribute describes a file format: which container ( for MP4), video codec ( for AV1, for HEVC and  for H.264), and audio codec ( for Opus and  for AAC) should be used.

Source: evilmartians.com