@pcwalton please rename this to `SELECTOR_WHITESPACE`, add a comment with this URL http://www.w3.org/TR/selectors/#whitespace , and remove the redundant definition in [line 524](https://github.com/pcwalton/servo/blob/173b55fc9b263c4657652c7f47ef30b75f429d0d/src/components/style/selector_matching.rs#L524).

The definition of class selectors refers to `~=` attribute selectors which refers to the above. The definition of the class attribute in the HTML points to [another definition of whitespace](http://www.whatwg.org/specs/web-apps/current-work/multipage/common-microsyntaxes.html#space-character), but it (fortunately) happens to be the same.