Checks the raw HTML source for structural mistakes the browser silently repairs: nested anchors and forms, block elements inside paragraphs, bare table rows, nested headings and unterminated attributes. These never appear in the parsed DOM, so they are invisible to tools that inspect a rendered page - but they mean the DOM you get is not the DOM you wrote.
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.