Particularly for mobile, the latest video-ad rendering is happening in JS [0]. This is because autoplay video is disabled by default on mobile, but video rendering in ASM JS / WebGL bypasses such restrictions. Yahoo [1] is doing this, for instance.

[0] https://github.com/brion/ogv.js

[1] https://hpto.yahoo.com/review/mobile/npte/a-haunted.html