Using content blocks with mixins (SCSS).

mixin-content.scss

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

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

.foo {
    @include keyframes(testAnimation) {
        from {
            opacity: 0;
        } to {
            opacity: 1;
        }
    }
    
    animation: testAnimation 0.2s ease-in-out;
}