Browsers have ceded way too much control to web designers. The user should be in control. When it comes to what fonts the computer uses, the text size, the color scheme, the user preference should be able to easily override the web site's code. Who's computer is it, anyway?

I'd be pretty over the moon if the browser supported the following preferences... especially given the number of electron or otherwise browser embedded UI options..

    system.font.primary
    system.font.header
    system.font.monospace
    system.colors.foreground
    system.colors.background
    system.colors.header
    system.colors.highlight
    system.colors.accent
It might be reasonable to have more than this, and the accent and highlight color may or may not be the same color... but it would go a long way towards matching the system defaults, with appropriate css variables injected as well.

The branding people have definitely won this war here. I agree with you, but the answer to your final question is, sadly these days, never the user or the supposed owner of the hardware. I think it’s pretty easy to argue today that when you boot a computer or phone, it belongs to Apple or Microsoft or Google. When you open a website or “app,“ the computer temporarily belongs to its developer. The fact that even browsers don’t have a built-in, simple-to-configure option to toggle persistent cookies on or off per website, opt-in, of course, is all the evidence you need of that for the web. None of this is OK with me, but it’s the world we have now.

The problem is that most fonts don’t support basic OpenType features. I make heavy use of small caps on my websites (they are IMO criminally underrated). If I were not using a custom font, most users would get hideous “synthetic” small caps.