Video acceleration and rendering acceleration are both mainstream.
On Chrome, open chrome://gpu which will tell you various things about its use of your GPU.
I'm not aware of an equivalent page for Firefox but opening about:support should somewhere inform you of the compositor, which when it says "WebRender" also makes use of your GPU.
Based on my previous knowledge, text, UI controls, video frames all have to go through the render pipeline. Should be no exception.
This is community talk, just fyi.
On Chrome, open chrome://gpu which will tell you various things about its use of your GPU.
I'm not aware of an equivalent page for Firefox but opening about:support should somewhere inform you of the compositor, which when it says "WebRender" also makes use of your GPU.