Sorry for the long delay in my reply but I wanted to give you specifics.
As my system is setup with 5.10-rc4-1 and mesa 20.2.2-3 and a 1920x1080 hdmi monitor, I do have v3d acceleration. Chromium 86.0.4240.198 recognizes v3d for webgl and webgl2. Firefox 83.0 recognizes v3d for webgl, but not webgl2.
When I visit this webgl example with 500 fish and 1024x1024 (the default): Chromium runs at ~15 fps and Firefox runs at ~10 fps.
When I run glxgears, it is in sync with the monitor at 60 fps.
Youtube videos run fine to my eye, but I suspect some frames do get dropped.
So in short, yes…v3d acceleration does work. However, I hope more improvements can and will be made.
Edit: I should add that I am using the lastest firmware, bootloader and dtoverlay=vc4-kms-v3d-pi4 in config.txt