[css-custom-properties] Document :host, :host-context styling, scoping

[kevinSuttle]

https://drafts.csswg.org/css-variables/
http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom-201/#toc-style-host

[tabatkins]

These are all defined in the Scoping spec. If you ever have trouble finding something, the CSS Index can often help with that; in particular, a Ctrl+F for ":host" would lead you right to where it's defined. ^_^