It is part of the spec, and is explicitly noted to avoid confusion starting in Selectors Level 3[0]: > Note: Repeated occurrences of the same simple selector a

It is part of the spec, and is explicitly noted to avoid confusion starting in Selectors Level 3[0]: > Note: Repeated occurrences of the same simple selector are allowed and do increase specificity. [0] https://www.w3.org/TR/selectors-3/#specificity https://www.w3.org/TR/selectors-3/#specificity