Why can a browser not be trusted for colour-accurate HDR review, and what controls exist?

Partially verified: the curator re-read S21; S20, S22, S24, S27 were not re-read by the curator and rest on the producer's pinned reading. The CSS Color HDR draft deliberately hides display headroom (tracking concern) and anchors SDR reference white at 203 cd/m2 for compositing; with UA tone/gamut mapping unspecified, two reviewers can see different results for identical bytes and no web API verifies what was displayed. Curator qualifications: Partially verified: the curator re-read S21; S20, S22, S24, S27 were not re-read by the curator and rest on the producer's pinned reading. Curator GET 2026-10-23 of the sources not re-read (mechanical source evidence, not semantic verification): S20: HTTP 200, bytes differ from the producer pin; S22: HTTP 200, bytes differ from the producer pin; S24: HTTP 200, bytes equal to the producer pin; S27: HTTP 200, bytes differ from the producer pin. Specs/docs read 2026-10-22; no display measured. HTML Standard zero-HDR grep (S27) and WebKit 26.0 dynamic-range-limit support (S24) from producer reading. Limitations: Specs/docs read 2026-10-22; no display measured. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://github.com/w3c/csswg-drafts/blob/3110cd02e77fa9e3869e433e1d40f615b6e611e4/mediaqueries-4/Overview.bs",
      "note": "css-color-hdr-1 Overview.bs at 3110cd02 L388-395 and L288-298"
    },
    {
      "url": "https://github.com/w3c/csswg-drafts/blob/3110cd02e77fa9e3869e433e1d40f615b6e611e4/css-color-hdr-1/Overview.bs",
      "note": "css-color-hdr-1 Overview.bs at 3110cd02 L388-395 and L288-298"
    },
    {
      "url": "https://github.com/w3c/media-capabilities/blob/f70a078df5eec5721b32273326d48f0b4901e514/index.bs",
      "note": "css-color-hdr-1 Overview.bs at 3110cd02 L388-395 and L288-298"
    },
    {
      "url": "https://webkit.org/blog/17333/webkit-features-in-safari-26-0/",
      "note": "css-color-hdr-1 Overview.bs at 3110cd02 L388-395 and L288-298"
    },
    {
      "url": "https://github.com/whatwg/html/blob/c6c3055c1287bf50070260d53555d8d6ab530771/source",
      "note": "css-color-hdr-1 Overview.bs at 3110cd02 L388-395 and L288-298"
    }
  ]
}