Early versions of Firefox shipped with a prominent stylesheet chooser in the status bar, that allowed alternate stylesheets on the page to be selected. Of course, hardly any website offered alternate stylesheets. So this later got moved to a menu and forgotten. According to this MDN page [1], IE8 has this feature too, which is wild to me, because that's right around the time most people accepted this feature as irrelevant.
Several browsers offer a way for the user to set a custom stylesheet that will be applied to every page. These options are almost always buried in some forgotten corner of an accessibility menu, if a UI is provided at all. The utility of this feature is passable when used to, say, enlarge text, or apply basic formatting to well-identifiable elements like h1, a, or table, and very useful for pseudoclasses. But because page layouts (i.e. the specific nesting of container elements, and the styles that affect them) between any two random websites can differ so widely, it's only capable of crude attempts to alter layout.
https://developer.mozilla.org/en-US/docs/Web/CSS/Alternative...