In level 1 and 2, physical units like mm and in were not tightly bound to px. UAs were expected to do their best to make stuff display at the correct size, which means they would have to know something about the screen (or printer), which might become confusing with multiple output devices. Anyhow, authors were used to assume a fixed relation between pixels and points (96:72 or 4:3), also they didn‘t understand that CSS px was not identical to their screens’ physical pixels. It made some sense to standardize the px:pt relationship, but the CSS WG decided to keep the pt:in ratio, too. Instead, they could have introduced a new point unit, e.g. pp. They didn’t, water, bridge, yada-yada, and now there is a fixed 3.78 px/mm ratio in level 3 (actually 96 px/in).
Today, CSS and its units are not only used to design websites, but also (mobile) app GUIs. On a touchscreen, the physical size of interactive widgets matters, because they are finger targets and (adult) human finger pads or tips are always in the same size spectrum, about 20±5 millimeters. Design guidelines use recommend different minimum sizes, also using different conversion factors.
I therefore see an important use case for actual physical units in CSS. It would confuse everybody and their dog if a future level of CSS Values and Units added “real” versions of mm, cm etc. with different (non-SI) abbreviations. A toggle like box-sizing is also inelegant. So I guess a new unit tailored to the actual problem to solve would be the best solution.
I’d like to see tip introduced, where 1tip equals, say, 20 millimeters (not 20mm). If it should be determined more exactly, I’d say make it the 4th quintile width of the dominant index finger, i.e. 80% of adults have smaller fingers. The height is smaller for finger tips, but about the same for finger pads; the thumb width is larger.
PS: It seems there had already been another proposal like this using touch as the moniker.
PPS: As it turns out, it was my 2012 self who proposed tip. I totally forgot about that. I’m also not sure I was the first or last to do so on www-style, since I haven’t found the alleged touch proposal yet.
