[css-syntax-3] Allow positioned {}-block in @property/initial-value

[cdoublev]

Per spec, a positioned {}-block is allowed for a custom property, but not for @property/initial-value:

8. [...] Otherwise, if decl’s value contains a top-level simple block with an associated token of <{-token>, and also contains any other non- value, return nothing. (That is, a top-level {}-block is only allowed as the entire value of a non-custom property.)

A possible solution could be to apply this only "if decls name is a supported CSS property".

However, I think the intention of step 9 is to allow reparsing a:hover {} / avoid reparsing --custom: var(1) {}, as a qualified rule. But it indifferently returns "nothing". So it could return an "invalid declaration error" instead. Consume a block's contents would also need to be updated accordingly.

[tabatkins]

Ah, I see, since initial-value isn't a custom property name, the parser will reject the value if it's got a {} block plus other stuff.

I'm not particularly sad about that. Allowing {} with other junk in a custom property is mostly there for back-compat. If you're registering a custom property, it's very likely to be with intent to use it for CSS stuff, where that sort of value would be completley invalid anyway. (You'd need to pair with a syntax: "*"; anyway, meaning that if you weren't using it for CSS values, the only reason to register it would be to turn off inheritance, which is a super minimal use-case at that point. Just put it in some parens or something if you really care, so it's not top-level.)

So, I'm inclined to just leave this as no change: you cannot express certain initial values for a registered custom property, even tho they are technically syntactically valid values for the property to have.

[cdoublev]

So defining it with ? would be a lie? I agree with your opinion on the use case, but I must admit that I am primarily interested in the design of CSS grammar.

I did not mention @function/result because it is not implemented anywhere but it applies as well.

[cdoublev]

I also did not mention that positioned {}-block in @property/initial-value are valid in Chrome and Firefox (non-exhaustive) today.

And if the intention is to early exit consuming nested rules in div { a:hover {} a:focus {} /* etc... */ } as a declaration, I presume that "grammar-aware" parser implementations would do it after consuming the declaration name.

(edit) They still need to abort parsing color: {} var(--custom) ... as soon as they see var(, or marker:hover {}, as soon as they see {.