[css-cascade] Do we need a keyword similar to `revert`, but for cascade layers?

[mirisuzanne]

Cascade Level 4 added the revert keyword for rolling back values to their definition in the previous origin.

In the draft Cascade 5 specification we provide cascade layers, which are conceptually similar to origins – but controlled by authors. It might be helpful to provide a similar syntax for rolling back to the value defined in previous layers.

[fantasai]

We should make sure that the preshint rules are given their own layer so that this works to unwind author rules without affecting preshint mapping. See #5836.

[mirisuzanne]

In another conversation, @fantasai suggested revert-layer as the keyword – and that makes sense to me as an initial proposal.

This would roll back the cascaded value of a declaration to the previous layer with a value specified, as if no current-layer rules were specified for this property on this element.

[css-meeting-bot]

The CSS Working Group just discussed CSS Cascade 5, and agreed to the following:

• RESOLVED: Add revert-layer to Cascade 5

The full IRC log of that discussion

<fantasai> Topic: CSS Cascade 5
<bkardell_> q-
<Rossen_> github: https://github.com//issues/5793
<fantasai> miriam: Cascade layers work like origins in that they stack on top of each other, containing specificity
<fantasai> miriam: with origins, useful to revert to previous origns
<fantasai> miriam: suggestion that this might be useful for layers
<fantasai> miriam: might be possible also to say which level to revert to
<fantasai> miriam: but that's the question here
<fantasai> florian: I think we need at least one
<fantasai> florian: if pple use and need more, can extend further
<bkardell_> Rossen_: my q was whether we would want to put switch stuff in that same spec for now
<argyle> q+
<fantasai> argyle: would be useful
<Rossen_> ack argyle
<fantasai> argyle: seems likely that I'd want to revert down to a particular layer
<fantasai> argyle: think of Island app want to import their 3 layers
<fantasai> argyle: might want to revert down to a particular one
<argyle> background-color: revert(bootstrap);
<fantasai> s/Rossen_: my q/Rossen_, my q/
<leaverou> what about revert just reverts to the result of every other layer being applied *but* the current layer?
<fantasai> miriam: I'm open to both
<fantasai> leaverou, that's * {all: revert} at the start of your style sheet
<fantasai> [missed]
<fantasai> florian: behavior is obvious, can spec both and see which ones have been implemented
<leaverou> fantasai: I thought that reverted the entire current origin?
<fantasai> Rossen_: ?? much easier to implement
<dlibby> q+
<fantasai> Rossen_: specific-level revert is easier
<fantasai> dlibby: for specific-layer revert, any concerns wrt footgun for authors
<fantasai> dlibby: if referring to specific layer not applied yet?
<fantasai> s/specific-level/revert-to-previous-layer/
<argyle> `background: revert(bootstrap, 'hotpink');`
<fantasai> Rossen_: can refer to these as target-revert vs immediate-revert
<fantasai> fantasai: current-level-revert
<fantasai> florian: Seems confusing
<fantasai> florian: targetted one, doesn't revert a particular layer, reverts all layers down to named one
<fantasai> florian: if there's nothing there, it applies styles from previous layers
<fantasai> dlibby: Florian might have answered, but if you didn't have layer with particular name / hasn't been applied, seems odd to target something might come later in the cascade
<Rossen_> fantasai: my take, let's do the immediate layer revert. It is simpler to implement and spec. Say cascade 5 includes this and build use cases from there.
<florian> +1
<Rossen_> fantasai: let's keep it simple and start from there
<Rossen_> q?
<Rossen_> ack fantasai
<astearns> perhaps a note with the possible extension and an ask for use cases for it?
<fantasai> sure
<fantasai> Rossen_: Current proposal is go with current-level-revert for Cascade 5
<fantasai> fantasai: proposed name is revert-layer
<fantasai> fantasai: if no immediate suggestions to rename, let's go with that
<fantasai> RESOLVED: Add revert-layer to Cascade 5
<fantasai> <br duration=10min>
<jfkthame> present-

[fantasai]

Forgot to tag 23e8ba6 and 090abe4 here. Anyway, that wraps up the edits, closing out.

[mirisuzanne]

I saw that the Chrome intent to prototype (from @xiaochengh) and platform status mention this as a part of the spec that they do not intend to implement, for complexity/performance reasons:

We will not implement the 'revert-layer' keyword due to its high complexity and likely negative performance impact.

I don't know if that's something we need to revisit in the spec, but I wanted to make sure that's documented here.