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