WebM

WebM (.webm) is a video file format developed by  Google. It is subset of the  Matroska container format developed by  Xiph, and incorporates VP8 encoding technology by  On2.

The biggest benefit of WebM over Bink (or AVI) is that of file size; a WebM video will be significantly smaller than that of a similar-quality Bink file. For example, the installation size of Hunt Down the Freeman decreased by 5.7 GB when switching its FMV cutscenes from Bink to WebM^[1].

WebM is supported in the following games, engines, and engine branches:

• Source 2
• Strata Source
• Counter-Strike: Global Offensive
• Hunt Down the Freeman

References

1. Hunt Down The Freeman - July blog: where we're at + looking for team members! - Steam News
   Bink video is replaced with webm (like CSGO and Source 2) and has successfully freed up 5.7GB of space