I used to work on custom embedded UIs that supported fractional scaling. This shit is hard.

You have to understand there's enormous effort that needs to go into this to make things look good. It's absolutely no surprise to me that Jobs era Apple used to stick to integer scaling ratios with relatively low-res phones while the competition battled with paper specs.

The trick to make things look good is to be mindful of the pixel grid, (and the subpixel layout). You have to choose font spacing and fonts so that major font features line up with the pixel grid. You sometimes have to slide a letter a bit to the left or right, which might result in inconsistent spacing, you might even want to have multiple versions of characters to hide these issues.

This applies to borders as well (both spacing and thickness).

Sometimes you can't make it look good no matter what you try - and the designer has to change it.

While flexbox and other super-duper layout algorithms might be very clever, if you use them, there's no way to line things up perfectly, and if you expect to, you might be in a world of hurt.

Adding PPI is a poor way to fix this. Even if you 2x the resolution, going from 1080p, to 4K, these issues still persist, and you quickly run out of hardware beyond that.

There's no wonder why modern 'flat' UIs usually have 1-2 pixel-ish gradients on edges of features, or use smooth transitions, it's a cop out - but that's the only thing that works with flexible layout, different pixel densities. But subtly, you notice things looking a bit blurry, just as if the images were low-res, even though the pixel density is insane.

ClearType is brilliant, but it only works with a certain set of assumptions, like a bit of light bleed (which exists on LCD but not on OLED afaik), and needs to know the subpixel layout, and can result in absolutely gorgeous looking fonts with relatively low res displays.

There's a reason why Windows 95-era UIs have a cult following - it's insane how sharp they looked even on hardware that on paper is much worse than modern stuff.