Because although I don't care much about the chrome, I sometimes have to use it. For example, the address bar in firefox is ridiculously small. Also, some apps, like firefox (again) have a weird adaptation of the scroll to the zoom. So if you zoom at 300%, it will scroll by a lot at a time, whereas 200% is still usable.

Also, 200% on an FHD 14" laptop means 960x540 px equivalent. That's too big to the point of rendering the laptop unusable. Also, X11 doesn't support switching DPI on the fly AFAIK, and I don't want to restart my session whenever I plug or unplug the external monitor, which happens multiple times a day when I'm at the office.

14 fhd is 157 ppi 24 4k is 184 ppi

This really isn't this far off. If we imagined the screens overlayed semi-transparently an 16 pixel letter would be over a 14 pixel one.

If one imagines an ideal font size for a given user's preference for physical height of letterform one one could imagine a idealized size of 12 on another and 14 on the other and setting it to 13 and being extremely close to ideal.

>So if you zoom at 300%, it will scroll by a lot at a time, whereas 200% is still usable.

This is because it's scrolling a fixed number of lines which occupy more space at 300% zoom notably this applies pretty much only to people running high DPI screens at 100% because if one zoomed to 300% otherwise the letter T would be the size of the last joint on your thumb and legally blind folks could read it. It doesn't apply to setting the scale factor to 200% nor the setting for Firefox's internal scale factor which is independent from the desktop supports fractional scaling in 0.05 steps and can be configured in about:config

layout.css.devPixelsPerPx