This, the same as dark mode, and media queries etc, all completely undermines the core idea of the web - a semantic representation of data, rendered by a user agent, according to the users preferences.

CSS & JS are the opposite.

Yes, I realize it's a losing battle, but it's a depressing thing nonetheless.

Dark Mode, and media queries are both just options/hints - they are implemented on the client side - by the user agent.

The actual problem is that most browsers have forgotten that they're "user agents" - and have stipped away all the configurations and options that they used to have, removing control from the user.

Vivaldi is still very configurable, and still behaves like the users agent.