postcss-custom-properties test for full support. Is this correct ?

input.css

:root,
.light {
  --color: black;
}

.dark,
.light--inverted {
  --color: white;
}

p {
  background: var(--color);
}

a,
.link {
  color: var(--color);
}

#important {
  --color: red
}

output.css

p,
.light p,
p.light {background: black;}

.dark p,
p.dark,
.light--inverted p,
p.light--inverted {background: white;}

#important p,
p#important {background: red;}

a,
.light a,
a.light,
.link,
.light .link,
.link.light {color: black;}

.dark a,
a.dark,
.light--inverted a,
a.light--inverted,
.dark .link,
.link.dark,
.light--inverted .link,
.link.light--inverted {color: white;}

#important a,
a#important,
#important .link,
.link#important {color: red;}

Here I think I got something http://jsbin.com/hunuy/2/edit