[css-sizing] Clarify how contain-intrinsic-size affects size containment algorithm

[Loirooriol]

CSS Containment has a full algorithm for sizing elements with sizing containment: https://drafts.csswg.org/css-contain-2/#containment-size.

But https://drafts.csswg.org/css-sizing-4/#explicit-intrinsic-inner-size just handwaves

If an element has an explicit intrinsic inner size in an axis, then after laying out the element as normal for size containment, the size of the contents in that axis are instead treated as being the explicit intrinsic inner size instead of what was calculated in layout, and layout is performed again if necessary. (If it has an explicit intrinsic inner size in both axises, this implies the first layout can be skipped.)

It's not clear:

• When it says "after laying out the element as normal", does this cover all steps of https://drafts.csswg.org/css-contain-2/#containment-size or only some of them?
• When it says "the contents in that axis are instead treated as being the explicit intrinsic inner size", why didn't we do this from the beginning? Why do a former layout as if there was no content?
• When it says "and layout is performed again if necessary", when it's not necessary?
• When it says "If it has an explicit intrinsic inner size in both axises, this implies the first layout can be skipped", why isn't it always the case?

[Loirooriol]

One example of an interaction which is not clear. https://drafts.csswg.org/css-contain-2/#sizing-as-if-empty says

Replaced elements must be treated as having an natural width and height of 0 and no natural aspect ratio.

A replaced element has no content, so is contain-intrinsic-size treated as the natural sizes (instead of 0 above)? Is the aspect ratio affected?

[css-meeting-bot]

The CSS Working Group just discussed Clarify how c-i-s affects size containment aglo, and agreed to the following:

• RESOLVED: contain-intrinsic-size size is used as the natural sizes of replaced elements (but does not set the aspect ratio)

The full IRC log of that discussion

<fantasai> Subtopic: Clarify how c-i-s affects size containment aglo
<fantasai> s/aglo/algo/
<fantasai> github: https://github.com//issues/7519
<fantasai> oriol: There's a bunch of sentences in the spec, mostly editorial, but the interaction of how the algo in containment spec is altered to take c-i-s into account
<fantasai> oriol: mostly editorial, might not worth it to discuss now, just one thing I wanted to discuss is
<fantasai> oriol: what about replaced elements?
<fantasai> oriol: The contain spec sais that when you have a replaced element with size containment, it behaves as if it had no natural size
<fantasai> oriol: and the natural size was zero
<fantasai> oriol: what happens if you have c-i-s?
<fantasai> oriol: what Chromium does is that the replaced element still doesn't have a natural size
<fantasai> oriol: but the natural width and height are the ones specified in c-i-s
<fantasai> oriol: does this make sense, or do we want some other behavior?
<fantasai> TabAtkins: having natural width/height take from c-i-s makes sense
<fantasai> s/had no natural size/had no natural aspect ratio/
<fantasai> s/have a natural size/have a natural aspect ratio/
<fantasai> TabAtkins: you asked, why do first layout if going to relayout
<fantasai> TabAtkins: answer is, we want to make sure that under normal circumstances, width calculates before height
<fantasai> TabAtkins: depending on contents, can have different effets
<fantasai> TabAtkins: once have width, then calculate height
<fantasai> oriol: so can we have an example of these cases in the spec?
<fantasai> ACTION TabAtkins add an example
<fantasai> ACTION: TabAtkins add an example
<fantasai> Rossen_: does that mean we don't need to change anything?
<fantasai> TabAtkins: Oriol's second example, needs fixing in the spec
<Rossen_> q?
<emilio> fantasai: we want to clarify that it doesn't set the a-r right?
<fantasai> Proposed: contain-intrinsic-size size is used as the natural sizes of replaced elements (but does not set the aspect ratio)
<iank_> some SVGs have this behaviour.
<emilio> TabAtkins: yeah
<emilio> fantasai: I wanna suggest that if the aspect-ratio...
<emilio> TabAtkins: we wanna say that it doesn't have an intrinsic aspect-ratio, the property can affect
<emilio> fantasai: wonder if there's utility in being able to set the a-r from those sizes
<emilio> ... guess it has to be opt-in, we don't know if the element is supposed to have an aspect-ratio
<iank_> if you want to opt-in to an aspect-ratio you can just set the aspect-ratio as well?
<emilio> ... we might want to have that ability
<emilio> ... yeah, you can do that explicitly, but it'd be easier to have it computed from c-i-s
<emilio> ... that's something to think about but can be addressed later
<emilio> Rossen_: objections?
<fantasai> RESOLVED: contain-intrinsic-size size is used as the natural sizes of replaced elements (but does not set the aspect ratio)