Over 2 years since the 120hz iPhone, Safari still doesn't support 120hz (testufo.com)

It wouldn’t matter if you had the option to install whatever browser you want, but Apple prevents you from doing so.

Edit: “fully support” might be more accurate, scrolling and such will be at 120hz, but it even something as simple as this test animation can’t run at full speed.

Doesn't appear to work in Firefox on Android either.

This is likely an intentional decision to save power.