Re: Adding support for HDR headroom in ScreenDetailed

Hi Christopher, All,

Thank you for the HDR headroom proposal — it’s on the Second Screen WG’s WIP TPAC agenda:

https://github.com/w3c/secondscreen-wg/issues/13


WG participants - please review the proposal and provide feedback via GH prior. See the links provided by Christopher for the issue(s), PR and explainer.

Color on the Web CG participants at TPAC are welcome to join us for this discussion.

Thanks,

-Anssi (Second Screen WG/CG co-chair)


On 23. Jun 2025, at 20.33, Christopher Cameron <ccameron@google.com> wrote:

Hello Second Screen WG (and ColorWeb-CG as FYI)!

The HDR headroom of a screen is a core parameter for how HDR content is rendered (see this description in CSS HDR<https://drafts.csswg.org/css-color-hdr/#introducing-headroom>). Advanced applications may want to perform custom tone mapping using this parameter. This is another brick in the HDR canvas proposal<https://github.com/w3c/ColorWeb-CG/blob/hdr_canvas_r2/hdr_html_canvas_element.md> that has been slowly making progress.

The short version of the proposal is: Add the screen's HDR headroom parameter to ScreenDetailed. It is a potential fingerprinting vector, so ScreenDetailed is a better fit than Screen. It can also fluctuate rapidly (e.g, when the screen brightness changes) and independently of other screen properties, and so it gets its own "on change" event, separate from the other screen properties.

Please see below for the fully fleshed out proposal:

  *   Issue<https://github.com/w3c/window-management/issues> on window-management API
  *   PR<https://github.com/w3c/window-management/pulls> on window-management API
  *   Explainer<https://github.com/ccameron-chromium/ColorWeb-CG/blob/screen_hdr_headroom/screen_hdr_headroom.md>

This has been shipping behind a flag for several years (although with some slight API differences, which have been ironed out to better fit the CSS HDR model). Now that HDR content is getting to be much more common, there have been several requests to get this out from behind the flag.