[web-animations-1] What should happen when duration of EffectTiming is a string that is not auto? #14206

The AnimationEffect::updateTiming method whose duration member is either a number or a DOMString. It is specified like this:

duration, of type (unrestricted double or DOMString), defaulting to "auto"
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.
In this level of this specification, the string value auto is treated as the value zero for the purpose of timing model calculations and for the result of the duration member returned from getComputedTiming(). If the author specifies the auto value, user agents must, however, return auto for the duration member returned from getTiming().

Neither this nor update the properties of an animation effect specify how DOMString values that are not "auto" should be treated.