[web-animations] CompositeOperation blend

[KevinDoughty]

I asked for a similar feature a dozen years ago in public-fx. There is obviously no interest but I’d like an issue to point to. Previously I used the keyword relative but blend is probably a better choice.

A subtract operation for CSS Values 5 (#10646) would be first, then CompositeOperation: blend for Web Animations. CSS Transition and CSS Animation additions could follow, as seen in the Firefox implementation shown here, along with blend:
https://phabricator.services.mozilla.com/D156634#5139009

blend in Chromium:
https://gitlab.com/kevindoughty/chromium-diff/-/commit/9238b5ae324d535390bbba35d165f45b101a5940

I actually got proof-of-concept working in WebKit more than a decade ago (barely, and pre Web Animations) but the code is not usable. CSS Transitions would look like this:
https://www.youtube.com/watch?v=lFgvb7p9dDs