[csswg-drafts] [css-animations] Allow percentages in animation-delay, relative to duration

LeaVerou has just created a new issue for 
https://github.com/w3c/csswg-drafts:

== [css-animations] Allow percentages in animation-delay, relative to 
duration ==
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.

It's a pretty minor change so I could make it myself if at least one 
of the editors approves. @tabatkins? @dbaron? @grorg?

Please view or discuss this issue at 
https://github.com/w3c/csswg-drafts/issues/558 using your GitHub 
account