Interpolation of prefixes on @keyframes in Sass

This simply isn't possible using variable interpolation. You can get around the @ assignment like this:

$at: unquote("@"); // either of these will work
$amp: #{"@"};

But then you end up with this error:

error sass/test.scss (Line 4: Invalid CSS after "": expected selector_comma_sequence, was "@-ms-keyframes ...")

Interpolation on @keyframes is not currently supported, but will be in the future. You can track the progress of that on GitHub. In the mean time, you'll have to write out your keyframe information by hand. A simple example looks like this:

@mixin keyframes($name) {
    @-webkit-keyframes #{$name} {
        @content;
    }

    @keyframes #{$name} {
        @content;
    }
}

Usage:

@include keyframes(foo) {
    0% {
        color: red;
    }

    100% {
        color: blue;
    }
}

A more complex example can be found in Eric Meyer's Sass Animation library.