[selectors] Define that the structual pseudo-classes never apply to pseudo-elements.

--HG--
extra : rebase_source : a4039d6fd422dea0b7628857e0d712433402eeb1
[css-images-3] Switch to Issue Tracking syntax.

--HG--
extra : rebase_source : fe40f17201eb11f9b07bc6e1f4df3e3b3735eeff
[css-will-change] Ack Benoit Girard.

--HG--
extra : rebase_source : 99b5ade781e26f8e2c6ee421ba824d3a9c8517b3