VP9 is an open and royalty free video coding format. ISO-BMFF (MP4) is an industry standard container and is broadly accepted and used in the industry. Chrome already supports the VP9 codec (currently only in WebM container) and the ISO-BMFF container. This enables them to be used together.
Docs: http://www.webmproject.org/vp9/mp4/
Samples: https://cs.chromium.org/chromium/src/media/test/data/bear-320x240-v_frag-vp9.mp4