[css-animations] Allow percentages in animation-delay, relative to duration #558
Description
A common use of animation-delay is using negative delays in conjunction with animation-play-stage to step through animations in order to debug them. This would be much easier if percentages could be used out of the box, instead of having to change the duration of the animation to 100s so that one could use "percentages".
In addition, even when delays are used in their intended way, they are often relative to the duration, so it could help DRY-up code.
No other property in the animation shorthand accepts percentages, so no disambiguation issues either.
It's a pretty minor change so I could make it myself if at least one of the editors approves. @tabatkins? @dbaron? @grorg?