IMO, browser makers failed us 15+ years ago by not making the ability to style/theme our sites more prominent as part of the UI. (and cookie management, but that's a different rant) The idea of using your own custom style sheets was always, at best, an extreme fringe feature. Yes, markup has to be 'right' for this to work, and browser makers catered to poor markup way too much in the early days. I'm not sure we can easily get out of it, but perhaps now... with a somewhat renewed focus on semantic markup and better CSS in browsers, we could encourage browsers to treat "user-specified" styling as a first class function of what it means to be a 'browser'?
Using Safari 5 right now, there's no option I can find to set a minimum font size. A 'default' font and size, yes, but that seems to be ignored or overwritten by every site I go to. It's 2012 and I still have to 'increase font size' on a number of sites I go to (and I know about keystrokes to do so - other people I know tend to just suffer with small fonts).