This is the rather small subset of CSS3 that is supported:
parse_selector(x)CSS3-like selector string.
Selectors:
Type selectors, e.g. input selects all <input> elements.
Class selectors, e.g. .index selects any element that has a class
of "index".
ID selector. #toc will match the element that has the ID "toc".
Combinators:
Descendant combinator, i.e. the space, that combinator selects nodes
that are descendants of the first element. E.g. div span will match
all <span> elements that are inside a <div> element.