It's difficult to pull this off with every website and keep it readable, without any hint from the site. This is why we have CSS for different screen sizes, rather than relying on mobile browsers to make all the decisions, along with an option to view the site in "desktop mode". I do agree that it would be annoying if sites made this decision for users.

But I think the more promising use for this could be enabling aesthetically pleasing color palettes that are power consumption optimized. Picture a HN that switches to darker tones of the current colors when you're on a mobile device.

I'm already constantly using "readability mode" to strip away as much of the custom styling as I can, because letting designers run riot - and building browsers that obey their wishes - has turned the web into such a mess. Tighter limits and more end-user control would be better. Different people have different preferences about their browsing experience and should be able to control that without having to rely on every last web designer to make helpful choices, because they really just don't.

My vapourware web browser is tentatively titled "fuck your web designer".

It will ignore site-provided CSS.

Interested?

Ha! I would like to give that a try.