[css-cascade-5] Proposal to disallow interleaving of @layer and @import rules

[xiaochengh]

Regarding empty @layer statements, the current spec says:

Such empty @layer rules are allowed anywhere either @import or other @layer rules are allowed.

And also:

Any @import rules must precede all other valid at-rules and style rules in a style sheet (ignoring @charset and empty @layer definitions), or else the @import rule is invalid.

This allows interleaving @layer and @import rules. I'm proposing to disallow it because:

• There doesn't seem to be a valid use case to interleave these two types of rules
• No interleaving makes it much easier to implement in Chromium. Otherwise, we need to maintain the relative ordering between @layer and @import rules in the StyleSheetContents class, which makes it more complicated without getting any benefit

The exact proposal is that rules in a style sheet must follow this ordering:

1. @charset
2. Empty @layer statements
3. @import
4. @namespace
5. All other rules + empty @layer statements

[mirisuzanne]

I don't think this would cause any functional problems - authors can still order layers as they want, and then add imported styles to those layers. But it does make the syntax for that a bit less flexible.

[tabatkins]

Yeah, no functionality is lost here, just a bit of flexibility. But I'd be okay with the restriction; it's probably better practice to put your @layer ...; rule either before or after your imports anyway.

[mirisuzanne]

Agenda+ to get a resolution

[css-meeting-bot]

The CSS Working Group just discussed disallow interweaving of @import and @layer, and agreed to the following:

• RESOLVED: @layer statements must occur before or after all @imports, not interleaved

The full IRC log of that discussion

<TabAtkins> Topic: disallow interweaving of @import and @layer
<TabAtkins> github: https://github.com//issues/6522
<TabAtkins> miriam: xiaochang is working on a cascade layers impl, and was concerned about allowing `@layers ...;`rules allowed anywhere in the doc, including between @imports
<TabAtkins> miriam: He's suggesting instead we just allow them before or after all the imports, but not between
<TabAtkins> miriam: I think that's fine; there's no functionality loss, just a little bit of flexibility. I'm fine with this.
<fantasai> wfm
<chrishtr> +1
<TabAtkins> Rossen_: Seeing support, any other opinions? Objections?
<emilio> q+
<chris> ping
<Rossen_> ack emilio
<TabAtkins> emilio: I'd argue we shoudl only allow after
<TabAtkins> emilio: The less syntax we have to mess with preload scanners the better
<TabAtkins> miriam: That actually does reduce functionality, we need to be able to order layers before we import them
<TabAtkins> emilio: That means the scanners need to deal with this
<TabAtkins> emilio: Do we also allow the block syntax?
<TabAtkins> fantasai: No, we disallow that intentionally
<TabAtkins> emilio: Okay, as long as there's no arbitrary blocks before the imports it's okay
<Rossen_> q?
<TabAtkins> RESOLVED: @layer statements must occur before or after all @imports, not interleaved

[fantasai]

@mirisuzanne Your edits also disallow interleaving among @layer and @namespace, is there a reason for that?

[mirisuzanne]

@fantasai That was a request from @xiaochengh, as I understand it had to do with the same parsing/performance concerns mentioned above. Maybe he can speak to it more here.

[xiaochengh]

Yes, I requested that the rules should follow the exact ordering I initially proposed.

The reason is that there's no loss of functionality, and the implementation in Chromium gets much simpler.