[css-values] unify CSS wide keywords and defaulting keywords

[yisibl]

These keyword definitions are currently scattered throughout the specification, making it easy for other CSS parsers or engine implementers to miss them. e.g.

• esbuild: https://github.com/sapphi-red/esbuild/blob/255a22fe22a98133da03c3a5df80238c91ad41b0/internal/css_parser/css_decls_font_family.go#L11-L15

var wideKeywords = map[string]bool{
	"initial": true,
	"inherit": true,
	"unset":   true,
}

• SWC https://github.com/swc-project/swc/blob/9d6a57c6e5954a6fcd35a55a27bf7067eec5fd9b/crates/swc_css_parser/src/parser/value/mod.rs#L1420-L1425

match &*value.to_ascii_lowercase() {
    "initial" | "inherit" | "unset" | "revert" | "default" => {
        return Err(Error::new(span, ErrorKind::InvalidCustomIdent(raw)));
    }
    _ => {}
}

• cssnano https://github.com/cssnano/cssnano/blob/2901a9c88ed5ac47c612e1b924669f28ae920347/packages/postcss-minify-font-values/src/lib/minify-family.js#L17

const globalKeywords = ['inherit', 'initial', 'unset'];

css-values-4

https://www.w3.org/TR/css-values-4/#common-keywords

Defined:

• initial
• inherit
• unset

css-cascade-4

https://drafts.csswg.org/css-cascade/#defaulting-keywords

Defined:

• initial
• inherit
• unset
• revert

css-cascade-5

https://drafts.csswg.org/css-cascade-5/#revert-layer

Defined:

• initial
• inherit
• unset
• revert
• revert-layer

css-values-4

https://www.w3.org/TR/css-values-4/#ref-for-css-wide-keywords

The CSS-wide keywords are not valid <custom-ident>s. The default keyword is reserved and is also not a valid <custom-ident>

There is a default keyword.

[tabatkins]

The definitions are already unified in the Cascade specs (and later levels replace preceding levels, so Cascade 5 is the canonical definition). Values merely mentions them, and points to Cascade for their definition; Fonts mentions them in an example (including the reserved "default" keyword)

[yisibl]

@tabatkins Thank you for your reply.

Yes, the CSS fonts spec mentions the terms common-keywords|CSS-wide keyword, so can we change them all to CSS-wide keyword in Cascade 5? Too many terms can be confusing and detrimental to communication.

Any identifier which could be misinterpreted as a pre-defined keyword in the font-family value definition, or the common-keywords|CSS-wide keyword values, is not allowed.

Also is it possible to add the "default" keyword to the Explicit Defaulting section in Cascade 5?

[fantasai]

Hi @yisibl, we've refactored the spec text around CSS-wide keywords in 9b95cab

For “default”, this is merely excluded from the <custom-ident> production, since it is not a valid CSS-wide keyword anywhere. We are also discussing adding additional keywords to the <custom-ident> exclusion list in #7431.

Closing out this issue, but let us know if there are any remaining problem.