Aha - CSS @supports
Use the new @supports rule to add fallback declarations for older browsers.
Write modern CSS and use the `supports not` to make it easier to delete the code once all browsers support a feature.

.open path  {
  d: path("M27.7 ... 4.8Z");
}

@supports not (d: path("")) {
  .open path  {
    --fill-color: transparent;
    --fill-color-active: currentColor;
  }
}