> Ideally I'd like to be able to set the CBR / VBR bitrate

What's wrong with the existing VBR/CBR modes? https://developer.mozilla.org/en-US/docs/Web/API/VideoEncode...

> I don't think you need a shader...

Ah I see what you mean. It'd probably be hard for us to standardize this in a way that worked across platforms which likely precludes us from doing anything quickly here. The stuff easiest to standardize for WebCodecs is stuff that's already standardized as part of the relevant codec spec (e.g, AVC, AV1, etc) and well supported on a significant range of hardware.

> ... instead of round-tripping into a CPU buffer

We're working on optimizing this in 2024, we do avoid CPU buffers in some cases, but not as many as we could.