This sentence should have a green background, not a red background.

"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.

Valid HTML 4.01 strict! CSS compliant