[css-cascade-5] Cascade Layers need CSSOM support

[anttijk]

CSSLayerRule interface needs to be specified.

[xiaochengh]

This is not entirely trivial. Since we have two types of @layer rules, the empty statement and the block rule, do we want the same interface for both of them, or different interfaces?

I don't have a strong preference. Just putting the options here for discussion.

---

If we use different interfaces, the following seems pretty straightforward:

[Exposed=Window]
interface CSSLayerStatementRule : CSSRule {
  readonly attribute CSSOMString names;
};

[Exposed=Window]
interface CSSLayerBlockRule : CSSGroupingRule {
  readonly attribute CSSOMString name; // empty string for anonymous layer
};

---

If we use the same interface, it would probably be like

[Exposed=Window]
interface CSSLayerRule : CSSGroupingRule {
  readonly attribute CSSOMString name;
  readonly attribute boolean isEmptyStatement;

  unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0); // Is this the correct syntax for override?
};

We need to override CSSGroupingRule.insertRule() so that it throws an exception when called on empty statements.

[tabatkins]

I think we should make them separate objects, since they have quite distinct data models - one vs N names, child rules vs not. They're the same name for authoring convenience, but that doesn't need to translate to them being the same object type.

[emilio]

I'd prefer not having two interfaces for this, fwiw... Also, for the @layer a, b, c; case we need a sequence of names, not just one name, right?

[tabatkins]

Yup, that's what I meant by "one vs N names". @xiaochengh's example supports that, it just exposes the N names as a single CSSOMString rather than as a list of them.

[anttijk]

Having two separate interfaces might actually make sense as there is nothing that can be shared.

CSSImportRule also needs an extension.

[emilio]

Yeah, I guess that's fine / alright, specially given we've given up on making CSSRule.type return something meaningful for these.

[anttijk]

On the other hand these interfaces are pretty useless anyway so I'm equally fine with a generic object that exposes nothing but cssText and the tree structure.

[emilio]

That'd be totally ok for me too.

[emilio]

FYI. Not ideal, but I needed some OM support to make DevTools etc work in Firefox.

What I did for now is implementing it as:

interface CSSLayerRule : CSSGroupingRule {};

Where cssRules will return null for statement layers. Again, probably not ideal, it was just the easiest to implement.