It's unclear how shadows should be drawn across various compositing operators

[Scony]

What is the issue with the HTML Standard?

It looks like https://html.spec.whatwg.org/multipage/canvas.html#drawing-model is not clear enough on how shadows relate to compositing operators and therefore browser implementations are somewhat random thus making a fiddle such as https://jsfiddle.net/np1e0uxd/ yielding different results in browsers I tried (no two browsers yield the same results):

• chromium
• firefox
• safari
• epiphany

[Kaiido]

The specs themselves seem particularly clear though: https://html.spec.whatwg.org/multipage/canvas.html#drawing-model and https://html.spec.whatwg.org/multipage/canvas.html#when-shadows-are-drawn

Firefox has an open bug for 10 years, and they have a special handling of a few composite modes which I'm not sure where it's coming from. I suppose WebKit has something like that too.

There seems to be 3 tests in WPT for shadow + composite, but they only use "xor" and "destination-out", which are not problematic in any browser. So I guess a first step would be to write more tests or extend these ones to use more (all?) composite modes.

cc @whatwg/canvas

[Scony]

Looks like I missed https://html.spec.whatwg.org/multipage/canvas.html#when-shadows-are-drawn - with that and https://html.spec.whatwg.org/multipage/canvas.html#drawing-model it all actually makes sense.

Looks like we need more WPT tests indeed!

Thank you for help.