It wasn't even really until the modern web that this happened; the browser is called the User Agent because it's acting on behalf of the user, not the website.

It wasn't even really until the modern web that this happened; the browser is called the User Agent because it's acting on behalf of the user, not the website. CSS Zen Garden showed what you could do just by changing CSS. Even today though you can still do what you want to a lot of sites, from adblocking, to restyling with things like Stylus, or full custom JS with Greasemonkey/Tampermonkey. It's the OS apps that are less customizable, but I don't want them to become even less so.