In level 2 of the web animations spec, times are no longer always
measures in milliseconds, they can also be percents when dealing with
progress-based (i.e. scroll-based) timelines.
We don't actually support percent times yet but this change will make it
easier to implement when we do.
This was added in Level 2 of the Web Animations spec
In this level of the spec, all time values associated with animations
(including this duration property) are no longer simple numbers but are
instead either percentages or numbers for progress- and time-based
timelines respectively. These values are represented internally by a new
`TimeValue` wrapper struct, and externally (i.e. in JS) as
`CSSNumberish`.
