A simple selector is either a type selector or universal selector followed immediately by zero or more attribute selectors, ID selectors, or pseudo-classes, in any order. The simple selector matches if all of its components match.

A selector is a chain of one or more simple selectors separated by combinators. Combinators are: whitespace, “>”, and “+”. Whitespace may appear between a combinator and the simple selectors around it.

The elements of the document tree that match a selector are called subjects of the selector. A selector consisting of a single simple selector matches any element satisfying its requirements. Prepending a simple selector and combinator to a chain imposes additional matching constraints, so the subjects of a selector are always a subset of the elements matching the rightmost simple selector.

One pseudo-element may be appended to the last simple selector in a chain, in which case the style information applies to a subpart of each subject.