[HTML] Fix divergence of CanvasRenderingContext2D and Offscreen with CanvasSettings

[ccameron-chromium]

こんにちは TAG-さん!

I'm requesting a TAG review of CanvasSettings.

There is a lot of duplicated spec text for CanvasRenderingContext2D and OffscreenCanvasRenderingContext2D, especially related to how CanvasRenderingContext2DSettings is handled. As with all things that are duplicated, there are accidental divergences.

This creates a new mixin interface CanvasSettings. This will unify the accidentally diverged paths, and also prevent this from happening in the future. It will introduce some features accidentally left out of OffscreenCanvasRenderingContext2D.

This is being pursued so that things like floating-point and HDR canvas do not need to add duplicate text when they are added to the specification.

• Explainer¹: https://github.com/ccameron-chromium/ColorWeb-CG/blob/canvas_cleanup/canvas2d_settings.md
• Specification: Fix accidental settings differences between CanvasRenderingContext2D and OffscreenCanvasRenderingContext2D whatwg/html#10904
• WPT Tests: https://chromium-review.googlesource.com/c/chromium/src/+/6157844
• User research:
• Security and Privacy self-review²: N/A
• Primary contacts:
  • @ccameron-chromium
• Organization/project driving the specification: ColorWeb-CG
• Multi-stakeholder support³:
  • Chromium comments: Support
  • Mozilla comments: getContextAttributes for OffscreenCanvasRenderingContext2D mozilla/standards-positions#1159
  • WebKit comments: getContextAttributes for OffscreenCanvasRenderingContext2D WebKit/standards-positions#445
• Status/issue trackers for implementations⁴:
  • Chromium: https://chromestatus.com/feature/5508068999430144
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1940414

Further details:

• I have reviewed the TAG's Web Platform Design Principles
• Relevant time constraints or deadlines:
• The group where the work on this specification is currently being done: ColorWeb-CG (because of floating-point and HDR canvas)
• The group where standardization of this work is intended to be done (if different from the current group):
• Major unresolved issues with or opposition to this specification:
• This work is being funded by: