Sometimes I want to: 1. size based on how big it is in the real world - mm (but accurate!) 2. size based on user preference - rem? 3. size based on how large

Sometimes I want to: 1. size based on how big it is in the real world - mm (but accurate!) 2. size based on user preference - rem? 3. size based on how large it appears to the eye - angle subtended on the retina (displays that don't know this can assume an average distance to the eye different for desktop, mobile, jumbotron, etc and work it out from their size) 4. size based on the kind of interaction the user will have (touch / mouse / eye / point) - some kind of new unit based on Fitts law - or should this just be a bunch of special cases? 5. size based on proportion of the available space - % vh vw I basically never need to size based on some other factors but maybe they have a use: 1. size based on the minimum resolvable size (i.e. the smallest something could be without losing definition). I guess this is much more of a concern if you're using a lot of very low resolution screens - px (but actual) 2. size based on the size of the whole document (including document that isn't visible) Of course this is all made extra complicated by the fact that sometimes containers need to be sized based on concerns bigger than them, while sometimes they're sized based on the sizes of their contents. And whatever framework you're using is probably bad at exposing key pieces of information about e.g. 'how much of this text can I get in this width' Unfortunately operating systems tend to have little clue about how big in the real world their display surfaces are, they are even less clueful about how far away the user is. Everything seems to be based on an early assumption that what was important was the dots on the screen, and as that assumption has proven false, instead of attacking the problem in a principled way, we've just created scaling fudges on top of pixels to take into account higher quality, user 'zoom' preference etc. In the hypothetical ideal world, I'd set my IDE editor font setting to a subtended angle setting and switch between desktop and laptop displays, upgrade the resolution of my screen, or the size of my screen and never have to change it.