[scroll-animations] should an infinite duration yield an exception for effects associated with a progress-based animation?

[graouts]

The WPT test scroll-animations/scroll-timelines/effect-updateTiming.html checks that setting Infinity for the duration of a keyframe effect associated with a progress-based animation throws:

// adapted for progress based animations
const gBadDurationValuesForProgressBased = [
  -1, NaN, Infinity, -Infinity, 'abc', '100'
];


for (const invalid of gBadDurationValuesForProgressBased) {
  test(t => {
    assert_throws_js(TypeError, () => {
      const anim = createScrollLinkedAnimationWithTiming(t,  { duration: invalid })
      anim.play();
    });
  }, 'Throws when setting invalid duration: '
     + (typeof invalid === 'string' ? `"${invalid}"` : invalid));
}

Web Animations Level 1 specifies duration thus:

The iteration duration, which is a real number greater than or equal to zero (including positive infinity) representing the time taken to complete a single iteration of the animation effect.

Reading through Web Animations Level 2 and Scroll-driven Animations Level 1 I cannot find any additional normative text that would indicate that Infinity is not a valid value for the duration of a progress-based animation.

Chrome throws in that scenario, so I expect it was the intention of Chromium engineers to specify this behavior.

[graouts]

Cc @flackr, @andruud, @kevers-google and @bramus.

[flackr]

Right, playing an infinite duration animation in reverse throws an exception because it would result in an impossible to compute state: https://drafts.csswg.org/web-animations-1/#playing-an-animation-section.

In a similar manner, playing an infinite duration animation with a non-monotonic timeline invokes a conversion that attempts to use the end time of the animation: https://drafts.csswg.org/web-animations-2/#time-based-animation-to-a-proportional-animation.

It's a good idea to call it out explicitly.