[css-variables] [selectors] Should custom properties work for :visited?

Should the following test-case be brown or yellow?

```html
<!doctype html>
<style>
  a {
    --foo: brown;
  }

  :visited {
    --foo: yellow;
    color: var(--foo);
  }
</style>
<a href="">Which color?</a>
```

Right now Gecko is yellow, blink / webkit are brown.