I always consider: That 9px text might look readable on your 4K iMac, but how about your users on a budget Chromebook with a 720p display? And text rendering differences between browsers and operating systems is still something that severely affects how the text looks, especially at thin font weights that seemed to be all the rage a few years ago.
yeah, chrome and safari supported retina displays on day one, but at yet another huge consistency nuclear bomb that we now have to remember forever.
It mostly works, but gets weird when you have non-scaled content in the design.
You don’t have non-scaled content in the design. Everything is scaled. <img width="32" height="32"> will be 32px (as in, CSS pixels) wide, whether that means 32dpx (device pixels) or 64dpx or something else.
(OK, so there are a few escape hatches; you have things like <img srcset> which allow you to choose different image sources for different scaling factors, and you can also query devicePixelRatio and change things based on it—though all of this isn’t actually guaranteed to map to physical pixels, e.g. my system uses 1.5× scaling but Firefox hasn’t finished implementing fractional scaling under Wayland so it renders at 2× and then scales down to 1.5×, so any line that is carefully made “one device pixel wide” will actually only be ⅔ of a device pixel.)
If you've ever run a mobile device without the real dimensions set properly (cheaper models, or custom roms) it gets interesting to say the least. Having less than stellar vision up close, I have every accessibility setting on my phone maxed... It's bad enough as it is the number of applications/sites that will render (in particular modals) off-screen.
In the end, however, nearly everything is scaled to either the real pixels, or often assume 72/inch as the actual screen density and scale accordingly. You really never know which.
My comment elsewhere in the thread is relevant on these units and how they’re anchored: https://news.ycombinator.com/item?id=36001822.