cssP's mission seems simple, but there's a lot to cover.
Off the tip of my tongue, here are some stuff that needs to be tested:
E:before /* simple */
E:first-child /* or */ E:nth-child(-n+10) /* pseudos with special characters */
E:before, E:after /* compound */
E:before, E, E:after /* compound pseudo with unrelated selector */
E:last-child F /* descendant selector inside pseudo */
E:first-child:before /* combined. Another one: E:hover:last-child */
:before /* implied star selector */
::-moz-selection /* or */ ::-webkit-scrollbar /* proprietary pseudos */