[css-contain] Clarify what happens when `display: contents` and `contain: style` are used together

[mrobinson]

Regarding https://drafts.csswg.org/css-contain/#containment-style:

The other types of containment have language like this:

If the element does not generate a principal box (as is the case with display values of contents or none), or its principal box is an internal table box other than table-cell, or an internal ruby box, or a non-atomic inline-level box, paint containment has no effect.

This means these types of containment have no affect when display: contents is used. In the case of contain: style this text isn't there. Should elements that have contain: style and display: contents establish a new style boundary? It seems that in Chrome it does, though perhaps with some bugs. In Gecko, this behavior might be a bit trickier to implement.

[emilio]

FWIW I think I think this should probably not work:

• Other counter properties do nothing on display: contents elements.
• Other containment types don't do anything on display: contents elements.

[tabatkins]

Yeah, I don't think the difference in wording was intentional here, it was just that the definition of the other containments is much more dependent on a box existing so we called out the effects.

I'm happy to say that no containment has an effect if you don't have a principal box.

[dholbert]

Could we get the spec updated to reflect this? CC @frivoal; maybe this needs to be on the CSSWG agenda?

Assuming we're in agreement, we need to update at least one WPT since the expectations have changed:
http://wpt.live/css/css-contain/contain-style-counters-002.html

Right now that test has the following note in an assert meta-tag (emphasis added by me):

In this test, the div#test does not generate a principal box because of 'display: contents'. Despite that particular condition, 'contain: style' will have an effect on div#test

If we change the spec as discussed above, then the test should be updated to expect that contain:style has no effect.

[dholbert]

[Ticking agenda+; I'm assuming this needs csswg discussion/resolution to move forward.]

[css-meeting-bot]

The CSS Working Group just discussed [css-contain] Clarify what happens when `display: contents` and `contain: style` are used together, and agreed to the following:

• RESOLVED: No containments apply to elements that don't generate a principal box

The full IRC log of that discussion

<TabAtkins> emilio: things like counters don't usually have an effect on things without boxes
<TabAtkins> emilio: it would be tricky/weird to have contain:style have an effect on display:contents elements
<TabAtkins> emilio: I think Tab agrees with this
<fantasai> TabAtkins: Agree this is fine. Just need to copy language over, and maybe generalize so that style containment doesn't apply to things without a box
<TabAtkins> astearns: concerns?
<TabAtkins> astearns: proposed resolution is to specify that style containment has no effect on elements without a principal box
<TabAtkins> TabAtkins: I think I'll just say that *no* containments work, but yeah
<TabAtkins> RESOLVED: No containments apply to elements that don't generate a principal box