What is the issue with the HTML Standard?

There is no way to indicate what tone mapping, if any, is done when calling functions such as drawImage. Adding a targetHdrHeadroom attribute to one of the mixins of CanvasRenderingContext2D would address this problem.

See explainer at:
https://github.com/ccameron-chromium/ColorWeb-CG/blob/master/canvas2d_hdr_headroom.md