Clarify blending isolation in 2D canvas

[sagudev]

What is the issue with the HTML Standard?

The spec just says:

Composite B within the clipping region over the current output bitmap using the current compositing and blending operator.

Which I understood as non-isolated blending, but it's actually isolated blending (per WPT tests). It would be nice to state that explicitly, as currently browsers are not respecting this (Firefox vs Edge):

[annevk]

WPT might be wrong as well. Please don't trust it blindly. Seems like something @whatwg/canvas should take a decision on and then clarify in the specification and tests.

[Kaiido]

That looks like a bug in Firefox with their path drawing methods. Using drawImage instead, they act as expected: https://jsfiddle.net/34xf27uh/

Ps: If you open an issue, it's caused by https://bugzil.la/1911231 With a skewed matrix, even fillRect produces the correct result.