a = IDs, b = classes/attributes/pseudo-classes, c = types/pseudo-elements
Selector 1 wins (higher specificity).
As per W3C Selectors Level 3, :not() contributes the specificity of its argument.
a = ID selectors, b = classes + attributes + pseudo-classes, c = type selectors + pseudo-elements. :not() uses its argument's specificity.
Compare (a,b,c) left to right. First higher value wins. Tie means source order decides.