[selectors] Is ::pseudo-element:empty valid?

Hi,

I'd like to match empty ::pseudo-elements, e.g.:

  ::before:empty {
    content: 'placeholder text while something asynchronously loads...';
  }

But ::before:empty doesn't seem to work for me.

Is this intended[1] or an implementation defect in Chrome?

-- Dan

[1] http://dev.w3.org/csswg/selectors4/#empty-pseudo