[HTML] Add OffscreenCanvasRenderingContext2D method getContextAttributes

[ccameron-chromium]

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvasRenderingContext2D

also

https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas/getContext

What specific section or headline is this issue about?

For OffscreenCanvasRenderingContext2D: add a method getContextAttributes

For OffscreenCanvas: "2d context attributes"

What information was incorrect, unhelpful, or incomplete?

In this PR to the HTML spec, we are adding getContextAttributes to OffscreenCanvasRenderingContext2D. Note that this method already exists in CanvasRenderingContext2D, and its omission from the spec was a mistake.

Additionally, we are unifying the creation parameters in CanvasRenderingContext2DSettings for both HTMLCanvasElement and OffscreenCanvas getContext. The desynchronized and willReadFrequently attributes were accidentally omitted from OffscreenCanvas getContext.

What did you expect to see?

In https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvasRenderingContext2D: Add documentation for the new getContextAttributes method

In https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas/getContext add documentation for the desynchronized and willReadFrequently attributes present in the HTMLCanvasElement getContext method.

Do you have any supporting links, references, or citations?

See this pull request and further links and discussion in this issue.

Do you have anything more you want to share?

No response