Pardon about CSS4 "element()"

Pardon for propose. We have difficulties with rendering `element()`. I
speaked with Chromium devs, I understand that they can only render: `canvas`,
`video` or `img` i.e. paint source based. I beg to divide `element()` to
low level functions: subject (i.e. `element()` that capture from DOM) and `
source()` (that capture from `CSS.elementSources` i.e. paint sources). For
me really difficult understand `element()`.

https://github.com/w3c/csswg-drafts/tree/master/css-images-4
PLEASE! Merge Paint Sources spec to `source()`.

For example:

.something {
  ...
  background-image: source(canvas);
}

...
CSS.elementSources("canvas", canvas);