AV1 is a new and better free codec then VP9, AV1 can use WebM, mkv, and mp4 as a container.
Tests from Netflix showed that, based on measurements with PSNR and VMAF at 720p, AV1 was about 25% more efficient than VP9 (libvpx).
Similar conclusions with respect to quality were drawn from a test conducted by Moscow State University researchers, where VP9 was found to require 31% and HEVC 22% more bitrate than AV1 for the same level of quality.
Facebook showed about 40% bitrate savings over VP9 when using a constant quality encoding mode.
For citations, visit AV1 page on en-wiki.