# What technical objections and independent re-implementations of APCA are recorded in the W3C wcag3 issue tracker?

- id: art_01M5MCTBZS6V5HMXBABJSY7RWH
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T12, T17 · tags: none
- updated: 2026-10-23T12:47:20.050Z
- url: https://veyrul.com/articles/art_01M5MCTBZS6V5HMXBABJSY7RWH

In w3c/wcag3 #226 a colour-spec contributor reports that APCA uses a plain 2.4 power instead of the sRGB transfer function (the curator recomputed a maximum deviation of 0.0252 relative luminance at mid-tones), then reports that an as-specified independent implementation matches the APCA demo's Lc values while CIE-luminance variants do not, so APCA is effectively sRGB-only; the APCA author replies that the 2.4 shaping is intentional and not standard luminance, and agrees to specify the shaping step separately.

In w3c/wcag3 #226 a colour-spec contributor reports that APCA uses a plain 2.4 power instead of the sRGB transfer function (the curator recomputed a maximum deviation of 0.0252 relative luminance at mid-tones), then reports that an as-specified independent implementation matches the APCA demo's Lc values while CIE-luminance variants do not, so APCA is effectively sRGB-only; the APCA author replies that the 2.4 shaping is intentional and not standard luminance, and agrees to specify the shaping step separately.

Curator qualifications: Pages/code read 2026-10-22; nothing streamed, emulated or rendered. Reproduction of Lc values is the implementer's report (comment 2583269331, 2022-06-22; read in comments JSON fc48ec518036cee4), not curator-executed. Issues 212, 221 and 29 (S14-S16) not re-read; no controlled readability study found there (producer). Rights/verification notes are shown as limitations (L00 0019/ADR-0011): they restrict reuse of the cited material or record what was not verified; they are not cleared by this summary.

Limitations: Pages/code read 2026-10-22; nothing streamed, emulated or rendered. [rights_conflict_record] Eligible with conflict; T17.03 (APCA scores are sRGB-specific; not a CIE-luminance metric).

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://github.com/w3c/wcag3/issues/226",
      "note": "w3c/wcag3 issue 226 body (GitHub API GET; bytes equal pin), opened 2022-06-16; curator recomputation from the sRGB piecewise EOTF (no source bytes)"
    },
    {
      "url": "https://github.com/w3c/wcag3/issues/212",
      "note": "w3c/wcag3 issue 226 body (GitHub API GET; bytes equal pin), opened 2022-06-16; curator recomputation from the sRGB piecewise EOTF (no source bytes)"
    },
    {
      "url": "https://github.com/w3c/wcag3/issues/221",
      "note": "w3c/wcag3 issue 226 body (GitHub API GET; bytes equal pin), opened 2022-06-16; curator recomputation from the sRGB piecewise EOTF (no source bytes)"
    },
    {
      "url": "https://github.com/w3c/wcag3/issues/29",
      "note": "w3c/wcag3 issue 226 body (GitHub API GET; bytes equal pin), opened 2022-06-16; curator recomputation from the sRGB piecewise EOTF (no source bytes)"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] In w3c/wcag3 #226 a colour-spec contributor reports that APCA uses a plain 2.4 power instead of the sRGB transfer function (the curator recomputed a maximum deviation of 0.0252 relative luminance at mid-tones), then reports that an as-specified independent implementation matches the APCA demo's Lc values while CIE-luminance variants do not, so APCA is effectively sRGB-only; the APCA author replies that the 2.4 shaping is intentional and not standard luminance, and agrees to specify the shaping step separately. · locator: w3c/wcag3 issue 226 body (GitHub API GET; bytes equal pin), opened 2022-06-16; curator recomputation from the sRGB piecewise EOTF (no source bytes) · source: https://github.com/w3c/wcag3/issues/226

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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