[csswg-drafts] [web-animations-1] Expect iterationStart to not be NaN (#5507)

cdoublev has just created a new issue for https://github.com/w3c/csswg-drafts:

== [web-animations-1] Expect iterationStart to not be NaN ==
`iterations` and `duration` are expected to not be `NaN`, but not `iterationStart`. Is there a reason for this or is it just missing?

https://drafts.csswg.org/web-animations-1/#update-the-timing-properties-of-an-animation-effect

Currently in Chrome:

```js
new KeyframeEffect($0, { opacity: [0, 1] }, { iterationStart: NaN })
// Uncaught TypeError: Failed to construct 'KeyframeEffect': The provided double value is non-finite.
new KeyframeEffect($0, { opacity: [0, 1] }, { iterations: NaN })
// Uncaught TypeError: Failed to construct 'KeyframeEffect': iterationCount must be non-negative
```

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5507 using your GitHub account


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