CSS Shadow 1: the exportparts form “::ident : outerIdent” is rejected by the rules for parsing part mappings

§5.3 defines the exportparts form ::ident : outerIdent and example #example-b11a0a88 uses ::before : preceding-text. §5.6.1 collects the leading token from code points that are not spaces or colons and returns an error when it is empty; ::before starts with a colon, so the token is empty, and §5.6.2 drops the mapping. The form defined in §5.3 can never be produced by the parser.

Consequence

exportparts="::before : preceding-text" forwards nothing, so a ::part(preceding-text) rule in the outer document never matches.

Proposed correction

Align the parsing rules of §5.6.1 with the pseudo-element form of §5.3, or remove the form and the example.

Issued by

World Wide Web Consortium (W3C) — CSS Working Group

Act

CSS Shadow Module Level 1 (Editor's Draft, 28 April 2026)

Place

§5.3 against §5.6.1 and §5.6.2

Record

Discrepancy

Class

Text of the act

Where it arose

The text of the act

Arxo's conclusion

Confirmed

Official text

Unchanged at last check

Last checked

2026-10-31

Evidence

Contradiction within the text. Quotations are verbatim from the official material; translations are ours and marked.

If ::ident is the name of a fully styleable pseudo-element, adds ::ident / outerIdent to el’s forward part name list. Otherwise, does nothing.

Collect a sequence of code points that are not space characters or U+003A COLON characters, and let first token be the result.

If first token is empty then return error.

If mapping is an error then continue to the next iteration of the loop.

How to check it without Arxo's tools

1. Open CSS Shadow Module Level 1, Editor's Draft of 28 April 2026 (W3C CSS Working Group) and find §5.3 Forwarding a Shadow Element: the exportparts attribute; §5.6.1 Rules for parsing part mappings; §5.6.2 Rules for parsing a list of part mappings. Compare the text with the quotation above. A copy is kept in this register: c84bdd0453ff210f-ed-css-shadow-1.html (SHA-256 c84bdd0453ff210fa95fb95b118d7a8d365f189ea8fab1ee7db791f199fe1db4, copy of the official file).
2. The finding holds if the quoted passages read as shown and the comparison gives the result stated at the top of this page. If it does not, write to us with the record ID.

How this finding was made

AI agents read the specifications and compared their sections. An AI coordinator checked the place against the full Editor's Draft and the Working Group's issue tracker, and two further AI models checked the report before filing.

Responsible institutions

• W3C CSS Working Group — maintains the specification

Reported to

Responses

No response recorded.

History

1. 2026-10-31Confirmed by Arxo
2. 2026-10-31Entered in the register
3. 2026-10-31Reported to W3C CSS Working Group (public issue tracker)

Cite this record

Arxo Law, Arxo Findings Register, ARXO-2026-0337, https://arxo.io/findings/ARXO-2026-0337/ (status as of 2026-10-31). Data: ARXO-2026-0337.json.