Interestingly, that does work on new wikipedia (good on them) but not on many other sites I tested and has some weird results in some places (for example, it doesn't change the regular text size on HN but makes the "reply" link larger). It seems this doesn't have the desired effect or support and could break things for general use. And unfortunately, as a web dev, I have to primarily work with default settings on.

But those sites would be broken according to spec, no? And certainly they won't satisfy any accessibility guidelines.