You are correct. I worked on this for years at Mozilla. See https://robert.ocallahan.org/2007/02/units-patch-landed_07.h... and https://robert.ocallahan.org/2014/11/relax-scaling-user-inte... for example. Some of the problems were pretty hard but the Web ended up in a pretty good place --- Web developers pretty much don't think about whether scaling factors are fractional or not, and things just work... well enough that some people don't even know the Web is "free-scaling UI"!

It mostly works but you can still run into issues when you e.g. want to have an element size match the border of another. Things like that that used to work don't anymore due to the tricks needed to make fractional scaling work well enough for other uses.

Why wouldn't it work? The border-size accepts the same kind of length units as height or width, no?

The problem is the rounding from fractional sizes due to fractional scaling to whole pixel sizes needed to keep things looking crisp. Browsers try really hard to make sure that during this process all borders of an element remain the same size, but this also means that they end up introducing inconsistencies with other measurements.

That's actually a different kind of scaling. The one at issue here is closer to cmd-plus/minus on desktop browsers, or two-finger zooming on phones. It's hard to make that look good unless you only have simple flat UIs like the one on this website.

They did make another attempt at it for apps with Dynamic Type though.

I'm certain that web style scaling is what the vast majority of desktop users actually want from fractional desktop scaling.

Thinking that two finger zooming style scaling is the goal is probably the result of misguided design-centric thinking instead of user-centric thinking.

At the time I was trying and failing to get Java apps to do it properly, so please send this feedback to whoever invented Swing in the 90s.

> misguided design-centric thinking

More like “let the device driver figure it out” - Apple is after all a hardware company first.

In terms of how its business works, Apple is primarily a fashion company.

A deeply technical one, yes, but that's not what drives their decision making.

User scale and device scale are combined into one scale factor as far as the layout / rendering engine is concerned and thus are solved in the same way.

The difference is developers are a lot more likely to have tested one than the other. So it's what you call a binary compatibility issue.

Similarly browser developers care deeply if they break a website with the default settings, but they care less if cmd-+ breaks it because that's optional. If it became a mandatory accessibility feature somehow, now they have a problem.