[css-counter-styles] Change override to extends. Add example.

--HG--
extra : rebase_source : 3eed1663aeb0007fe4e0d73fabfe2dfa92c3d61e
[css-font-loading] Replace [SetClass] with an explicit listing of Set methods, ugh.
[css-font-loading] Add issue about the FontFaceSet status promise.
[css-font-loading] typo