Reference:
"since it is not known if the comma may acquire other
meanings in future updates of CSS, the whole statement should
be ignored if there is an error anywhere in the selector, even
though the rest of the selector may look reasonable in CSS 2.1."
CSS 2.1, Section
4.1.7 Rule sets, declaration blocks, and selectors
NS 6.1, NS 6.2, NS 7.0, NS 7.1, NS 7.2, Seamonkey 1.x, K-meleon 1.x, Firefox 1.x, Firefox 2.x, Safari 2.0, Safari 3.0.2, Opera 9.x, Amaya 9.54, Hv3 TKHTML alpha 16, Konqueror 3.5, Galeon 2.0.2, Epiphany 2.14 all pass this test.