In level 1 and 2, physical units like
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
ptrelationship, but the CSS WG decided to keep the
inratio, 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
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.