[css-unit-au] Support for "au" CSS sizing unit

[boutell]

See:

https://www.w3.org/TR/css-values-3/#absolute-lengths

While excellent for screen and print use, these units are impractical for measurements on an astronomical scale, e.g.:

https://xkcd.com/2863

This would be a new "absolute length" unit, named au, per section 5.2 of the spec.

For purposes of the equivalence table, au can be listed as 1.496e+13 cm.

[bleper]

😒🤨 https://drafts.csswg.org/css-egg-1/#astro-units

[Loirooriol]

Concern: "au" means "astronomical unit", but there is potential confusion with "app units" https://docs.rs/app_units/latest/app_units/

Also, one astronomical unit is not that big, so would it be better to add kpc (kiloparsec) instead? See #2491 (comment)

[boutell]

Hmm. Good points. Also "au" incorporates "unit," but "unit" is implied in this context. Perhaps just "a" for Astronomical (unit)?

[tabatkins]

[checks calendar] hey, it's not April yet

[aersida]

au can also be confused with:

• Gold - so are we talking about mass here, so is this really about some kind of font weight? or
• Australia - which is also very large (but not as big as 1 Astronomical Unit), and mostly empty