Selectors Level 3 only allowed :not() pseudo-class to accept a single simple selector, which the element must not match any of. Thus, :not(a, .b, [c]) or :not(a.b[c]) did not work. Selectors Level 4 allows :not() to accept a list of selectors. Thus, :not(a):not(.b):not([c]) can instead be written as :not(a, .b, [c]) and :not(a.b[c]) works as intended.