Do we really want functions in CSS? I guess that ship has sailed a while ago, since CSS reached Turing completeness.

  /* Define a function: */
  @function --gradient-to-transparent(--color, --direction: bottom) {
    result: linear-gradient(
      to var(--direction),
      var(--color),
      transparent;
    );
  }

Why not provide better integration with JavaScript, to make this kind of acrobatics unnecessary.