I have a WIP patch that does, I think, plumb the CSS parser mode [1]. I'm still not seeing media controls change with it applied, however. I also tried forcing a crash whenever we queried whether the immersive media feature is enabled, and it was never tripped. It does look like your CL has landed, though [2]. Am I doing something wrong?

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1002152
[2] https://chromium-review.googlesource.com/c/chromium/src/+/993396

Do you have modern media controls enabled?

I did not, embarrassingly. :)

With my patch, we do indeed parse the new immersive controls scheme, BUT we always apply the immersive styles and I'm not sure why that is. I have a test page which changes color in and out of immersive mode, and it works correctly (if I hack up the code to ignore the UA stylesheet restriction, of course), but we don't seem to reevaluate in this case. Still looking into why..