I have a record player with a system like this for confirming that the speed is accurate. The turntable has a few rows of "dots" for different rpms and there's an LED light which flashes at a certain speed that then makes one row look more stable than the others, indicating the speed.
If anyone sees a static "image" on their iPhone, check that you have Reduced motion disabled in system Settings. I totally forgot I did that and just assumed it doesn't work on mobile.
On my phone, as long as the page is scrolling, it seems to indicate 120 Hz, but after a few seconds there is a sudden switch to 60. Not sure if the tool is working correctly.
That'll be one of the tricks your browser employs to save compute time on animations for inactive tabs/windows. Interact with it again and the browser will allow scripts/animations to run at full speed again.