WPT details: There are already some tests that test functionality in offscreencanvas and imagebitmap rendering context. This is the link(https://github.com/web-platform-tests/wpt/pull/16032) with the new web-platform-tests.
We are adding ImageBitmapRendererContext as a valid context for OffscreenCanvas. ImageBitmapRenderingContext allows direct transfer of an ImageBitmap to the canvas surface. This is particularly important as a way to render an ImageBitmap to a Canvas from a Worker (without transferring the ImageBitmap to the main thread). This is also currently the spec-defined way to render WebGPU content to the screen.
We want to be able to move away from CPU to GPU the operations involving ImageBitmap rendering context. This is also currently the spec-defined way to render WebGPU content to the screen.
Docs: https://html.spec.whatwg.org/#the-imagebitmap-rendering-context https://html.spec.whatwg.org/#the-offscreencanvas-interface
Explainers: https://github.com/whatwg/html/pull/4455