Well, there is yt-dlp, if you count that as a browser. It has hacks for downloading from nearly every website that has weak DRM. It also has a fallback for guessing how to download from arbitrary websites.

If you mean drm'd content, I think that's hopeless now, the security is at the hypervisor level nowadays. I can't even play that content on my old vga monitors. For the average youtube video, I don't see why not in principle.

"after all decoding" isn't desirable because then you have to re-encode it, incurring generation loss.