1740584 - Add new relative units RCH, REX, RCAP, and RIC

Status: NEW

(Core :: CSS Parsing and Computation, enhancement, P3)

Description

[Bill Goldstein [:aja] (UTC-6)]

Expected results:

Add new relative units RCH and REX

Comment 1

[Jonathan Kew [:jfkthame]]

Also ic and cap, and their root-element variants ric and rcap.

Comment 2

[Emilio Cobos Álvarez (:emilio)]

This should be sort of straight-forward to implement following the rem code. Just adding more root font metrics in Device and tracking changes and so on.

Comment 3

[Jonathan Kew [:jfkthame]]

I'm not sure we want to exactly follow the rem pattern here. AFAICS, for rem we always record the root font size in Device during finish_restyle for the root element, which is OK as it's quite simple -- it's just the computed value of the font-size property. But retrieving the other metrics ch, ex, ic and cap, is significantly more work as it requires actually instantiating a font to read metrics from it. I suspect the root-element versions of these units will only rarely be used, so it seems regrettable to always do that work.

Can we instead do something like evaluate them lazily on first use (which will usually not happen), and only then cache the value in Device?

Comment 4

[Emilio Cobos Álvarez (:emilio)]

We can, though is it really that much work to get the initial font-size metrics for the root element? In particular, won't the initial font metrics ~always be used for layout at some point? Anyways doing it lazily seems doable as well.

Comment 5

[Jonathan Kew [:jfkthame]]

I'd be curious how often the root element's font metrics get used for anything; I'm guessing it may be quite common that a site specifies a bunch of font-* properties on the body, and nothing actually refers back to or uses the root's font, but I haven't looked into this deeply.

Comment 6

[Thomas Wisniewski [:twisniewski]]

Safari and Chrome now support all four of these units as of WebKit preview 179.

Comment 7

[Michal Čaplygin [:myf]]

(In other words:) According MDN CSS length BCD, Firefox is the only* major browser currently lacking support for rcap, rch, rex and ric units, as of now.
(* Samsung Internet reportedly does supports neither cap nor rcap.)