CSS Animations 1: “the last one in document order wins” for same-name @keyframes does not account for cascade layers

CSS Animations 1 §3 says that of several @keyframes with the same name the last one in document order wins. CSS Cascade 5 §6.4 says @keyframes inside cascade layers use the layer order to resolve collisions, as the Working Group resolved in issue 6404 in 2021. The sentence in CSS Animations 1 was not updated.

Consequence

Cascade 5's own example #example-14540e6d: by Cascade 5 the sidebar animates translate from the override layer; by CSS Animations 1 it animates margin-left from the framework layer, which comes later in the document.

Proposed correction

Let CSS Animations 1 §3 defer to the layer order and keep document order within one layer.

Issued by

World Wide Web Consortium (W3C) — CSS Working Group

Act

CSS Animations Level 1 (Editor's Draft, 13 September 2026)

Place

§3, against CSS Cascade 5 §6.4

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

Reproduced by execution. Quotations are verbatim from the official material; translations are ours and marked.

If multiple @keyframes rules are defined with the same name, the last one in document order wins, and all preceding ones are ignored.

At-rules (such as @keyframes, @font-face, @page, etc.) that are defined inside cascade layers also use the layer order when resolving collisions or cascading.

How to check it without Arxo's tools

1. Open CSS Animations Level 1, Editor's Draft of 13 September 2026 (W3C CSS Working Group) and find §3 Declaring Keyframes. Compare the text with the quotation above. A copy is kept in this register: a558664f18fc69e9-ed-css-animations-1.html (SHA-256 a558664f18fc69e909978b802a2cbf5a865897f2eb1f1c02bed6b2845642dfff, copy of the official file).
2. Open CSS Cascading and Inheritance Level 5, Editor's Draft of 13 April 2026 (W3C CSS Working Group) and find §6.4 Cascade Layers. Compare the text with the quotation above. A copy is kept in this register: 2c51fc71d5bfec21-ed-css-cascade-5.html (SHA-256 2c51fc71d5bfec21aee9acf5004344fc0b5197db31fc48a1ae5373f3fbc4670f, copy of the official file).
3. 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; the example was also run through executable rules of both readings. 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-2026-0339, https://arxo.io/findings/ARXO-2026-0339/ (status as of 2026-10-31). Data: ARXO-2026-0339.json.