I was surprised, too. But it did work on Raspbian buster minus one.

Then I upgraded to Raspbian buster to get a more recent version of Chromium since HBO Max was blocking me based on whatever version shipped with (buster minus one).

At that point I started shooting various startup flags at Chromium from the web of wizards' incantations until video started to play again. That including removing the 'f' per the incantation I mentioned. This cured video playback health but it put my mouse under some kind of latency spell.

I think I could beat this level with a drowsy mouse if only I could find increased magic for the subtitles somewhere.

Does anyone know if there is a Chromium arg I can use to just farm some magic?

Laggy mouse is the surprising part I think. It used to be that you could count on the mouse still moving smoothly even when the rest of the system was falling to pieces. I understand this was because the mouse was a hardware-rendered sprite controlled by hardware interrupts. I don't claim to understand the details, but my impression is things have changed a lot.

The mouse lags. Sometimes it even turns on an invisibility cloak which I didn't even know was possible.

I need to see if there are any RPI400 Chromium Video Acceleration Challenge players streaming on Twitch, maybe they'd have some clues. There are bound to be a bunch of useful weapons to beat this level but I don't know how to unlock them.

Like someone else mentioned it's probably because the video is in some kind of pure framebuffer mode.