仕様書（http://www.w3.org/TR/REC-CSS2/selector.html）より

擬似要素はサブジェクトの後ろにしか付けられない。サブジェクトとは、セレクターと一致するＨＴＭＬの要素のことか（a要素、li要素、等々）

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.