Now that it's possible to use ::distributed to style the nodes distributed to a given insertion point, the spec needs to clarify how this can be used to make selectors that match starting from distributed nodes themselves.

In general, one wants to be able to style the distributed nodes using complex css selectors:

/* style match .bar with .foo matching ancestors */
content::distributed(.foo .bar) {
  ...
}

However, it's common to want to create selectors that start at the distributed nodes themselves. The :root selector could be use for this purpose (http://dev.w3.org/csswg/selectors4/#the-root-pseudo).

/* match .bar descendants of distributed children matching .foo */
content::distribute(.foo:root .bar) {
  ...
}