I came to write this. It's a nice experiment anyways.
// Generate keyframes
// Sass interpolation doesn't work with keyframe. CF : https://github.com/nex3/sass/issues/46
// so I use a tricks ==> "appear-'#{$i}'" - And it doesn't work in Firefox.
It's impossible to generate @-moz-Keyframes + interpolation with Sass yet. That's why the experiment works only in Webkit.