[css-fonts] font-size Descriptor for ex Unit

[Crissov]

There currently is no font-size descriptor for use in @font-face. I propose it be added to manually specify the x-height and hence determine a non-fallback size (i.e. 0.5em) of the ex unit, which would be more useful and intuitive than the font-size-adjust property. If cap was added as a unit for the height of capital letters #660, this descriptor should also be able to set it. I think it makes most sense to only allow percentages of 1em.

@font-face {
  font-family: foo; src: url(foo.woff); 
  font-size: 45% 90%; /* fallback/default = 50% 100% */
}
.foo {
  font-family: foo; 
  font-size: 20px;
  padding-top: 1ex; /* = 0.45em = 9px */
  padding-left: 1cap; /* = 0.9em = 18px */
}

[Crissov]

It might make sense to als include tuning of the (usually horizontal) ch unit, which is proposed to be affected by font-size-adjust in #750. This may mean that the font-size descriptor should be split into a font-height and a font-width descriptor (or become a shorthand for those).

[litherum]

See also: #806

[svgeesus]

This seems to be covered by the improvements to Relative sizing: the font-size-adjust property @Crissov do you agree?

[Crissov]

Hm…

Above, I proposed to specify, in a descriptor, a fallback ratio for ex as a percentage of em if the actual first available font value is not available. The newly extended font-size-adjust property value syntax still requires the actual value to be available when <number> is used and, of course, also when from-font is used instead.

In conclusion, I agree that this looks very similar, but could actually work in combination: the descriptor would provide a fallback value for m′, while the property specifies m.

PS: If a descriptor was added, the correct answer to #6384 could be affected.