less.js/test/less/mixins-important.less

.mixin (9) {
  border: 9 !important;  
}
.mixin (@a: 0) {
  border: @a;
  boxer: @a;
}

.class {
  .mixin(1);
  .mixin(2) !important;
  .mixin(3);
  .mixin(4) !important;
  .mixin(5);
  .mixin !important;
  .mixin(9);
}