apply-properties.css

/* Either external library of these helper rules exist */
/* Or can be added to any stylesheet */

--big {
  font-size: xx-large;
}

--red {
  color: #c00;
  background: white;
}

--basic {
  -webkit-appearance: none;
  appearance: none;
  border: none;
}

/* Then you use them with an --apply property */

button {
  font-family: impact;
  --apply: big red basic;
}

/* And end up with this output, and no helper rules */

button {
  font-family: impact;
  font-size: xx-large;
  color: #c00;
  background: white;
  -webkit-appearance: none;
  appearance: none;
  border: none;
}