[web-animations-1] Expect iterationStart to not be NaN

[cdoublev]

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:

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

EDIT: and same question for all properties of EffectTiming or more exactly, why are they not validated?

[birtles]

iterations and duration are expected to not be NaN, but not iterationStart. Is there a reason for this or is it just missing?

Yes, iterations and duration have type unrestricted double so they can take values like Infinity and NaN. That's because those values need to be able to represent infinity.

iterationStart, however, has type double so it can't take such values. Validation that iterationStart is not NaN happens at the WebIDL binding layer and so no additional spec text is required.

new KeyframeEffect($0, { opacity: [0, 1] }, { iterations: NaN })
// Uncaught TypeError: Failed to construct 'KeyframeEffect': iterationCount must be non-negative

(That's odd that Chrome reports an error for iterationCount when validating iterations.)