Re: [csswg-drafts] [cssom] Proposal: CSS.assign (#4486)

> It would be lovely to have the ability to define multiple CSS properties at once, without causing reflows or repaint during each property that might trigger those.

Doesn't this happen already? AFAIK, unless you force it, styles are not applied synchronously.

For example, if I go to https://drafts.csswg.org/cssom/ and run
```js
for (let i = 0; i < 100; ++i) {
  document.body.style.width = Math.random() * 1000 + "px";
  document.body.offsetLeft;
}
```
it takes like 8 seconds, because I'm forcing layout at each iteration. But if I remove the `offsetLeft` line, it's a single millisecond.

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