<===> input.scss
@mixin will-change() {
  @supports (will-change: transform) {
    will-change: transform;
  }
}
div {
  a {
    top: 10px;
    @include will-change();
  }
}

<===> output.css
div a {
  top: 10px;
}
@supports (will-change: transform) {
  div a {
    will-change: transform;
  }
}