Does not support variables

[kaisellgren]

It looks like variables are not supported:

a {
  --base-hue: 240;
  --whiteness: 60%;
  --darkness: 25%;
  color: hwb(var(--base-hue), var(--whiteness), var(--darkness));
}

The above code produces:

Unable to parse color from string: hwb(var(--base-hue), var(--whiteness), var(--darkness))