> I would expect two large groups of user behavior:

I think you're missing:

3. People who set the font size once, when they configure their browser, to their preferred font size and expect, for some weird reason, that this will be respected!

> The tool to take control of all of that is reader mode, which is a great option when a website has screwed up its typography.

That would be great, if there wasn't a huge correlation between websites that screw up their typography, and websites that use an ungodly mess of nested `div`s and javascript-based content rendering that breaks reader mode.

On Firefox there's a preferred, minimum and maximum sizes. If you want sites to respect it, you set the minimum, otherwise it's a suggestion.

That third group is small enough that if it is impractical to properly support all three, they are the ones that will be ignored first.

I set the text size on my mobile browser. You'll understand as you get older and your devices keep getting more pixels.

I do too, and I’m in group #2. Are you saying you’re not?

No, I set it one for each device, then I never touch it again. So I'm am the third group that you doubt exists.

I don't want to fiddle with it. I read text on my phone, I have a preferred size to read, I shouldn't have to change it more than once. The few sites which end up too big or too small I read begrudgingly. Can I even set font size per site?

Pinch zoom doesn't work because now I have to pan left and right for each sentence?

I'm totally with Karellen on this.

At least in safari, you can hit the AA button and increase the font size there.

All CSS font sizes are already responsive because CSS pixels are an abstract concept. Adding your own hacks on top of that instead of respecting system settings is counter-productive.