Every inline and linked stylesheet is parsed. A parse error means the browser discarded a declaration or a whole rule at load time, so styling that looks correct in the source never applied.
Impact is importance multiplied by the confidence tier’s weight — SPECIFICATION 1.0, RESEARCH 0.7, EMERGING 0.3, EXPERIMENTAL 0.1. Two rules backed by the same class of evidence therefore always carry the same weight, which is what makes the score reproducible rather than hand-tuned.
Built at scan time by stylesheet conformance, not from our static registry. That engine evaluates its whole rule set in one pass, so this check cannot be run on its own — it runs as part of a scan.