Repeat animation-delay throughout iterations

Something I run into quite a lot as I build animations, is that animation-delay doesn't persist throughout iterations.

Take the following code:

div {
  animation: unicorns linear infinite forwards 5s 3s;
}

On the first iteration it would wait 3 seconds before it starts, then it plays for 5 seconds. On consecutive plays it will loop the 5 second part but skip the delay. This is certainly useful in certain cases, but there are also cases where you want the delay to persist; kind of like `animation-fill-mode: forwards;`.

I propose a second argument for the `animation-delay` property: repeat.

div {
  animation-delay: 3s repeat;
}

This will ensure that the delay is repeated on consecutive iterations. Alternatives to the repeat keyword are welcome, simply chose it because it is already in CSS vocabulary.

Bugzilla report: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28936

—Michiel