I don't understand how one can design a website that would survive arbitrary style changes. I think that is unrealistic, so the designer should expect all default styles to be standard. And if the user has too much free time to change the font size then it is their own problem; my suggestion is that they simply use reader mode and change the styles there.
I would like to remind that some time ago browsers allowed to change the default font size; it never worked well so Opera started to scale the whole page instead. Other browsers followed it.
Android browsers seem to repeat the same mistake by the way: they override developer's styles when the user changes font size in OS accessibility settings.
I mean, it's my computer. I should ultimately be in control of how a document renders. I might need larger text due to poor eyesight, or need to use a screen reader. Or I might just be irritated that the web developer just up and decided that light gray on dark gray text looked cool (it doesn't, it barely can be read). Or I might want to scroll with my keyboard because a mouse is painful to my RSI. If I go out of the way and set up accessibility settings, I would expect all applications on my device (including the web browser) to respect those settings.
I would like to remind that some time ago browsers allowed to change the default font size; it never worked well so Opera started to scale the whole page instead. Other browsers followed it.
Android browsers seem to repeat the same mistake by the way: they override developer's styles when the user changes font size in OS accessibility settings.