For a small taste,
"Meet Leon - superfast GPU-accelerated (WebGL) mpeg1-like video decoder in JavaScript"
https://www.easy-bits.com/mpeg1video-decoder-webgl-gpu
See mpeg1video-decoder-webgl.js on developer tools.
I didn’t realize enough of the work in decoding mpeg was paralellizable enough to do on the GPU, let alone in the constrained world of WebGL, but apparently it is.