Medium impact · weight 7 · worth up to 1.9 points of the total score
Browsers repair broken markup silently, and each one repairs it slightly differently. Unclosed and misnested tags are where a layout that works in one browser collapses in another.
Fix the reported positions: usually an unclosed tag, a stray closing tag, or an attribute missing its quote.
Check a page for this, and the other 75 checks at once
Element ids are unique
An id must appear once per document. Duplicates break label-to-input pairing, in-page anchors, and any script that looks an element up by id, and every one of those failures is silent.
Page is served over valid HTTPS
Browsers mark plain http pages as not secure, and an expired certificate replaces the whole page with a warning that almost nobody clicks through. Certificates now last 90 days or less, so a renewal that silently stopped working takes a site down within weeks. Protocols older than TLS 1.2 are refused by current browsers.
Character encoding is declared
With no declared charset the browser guesses, and the guess is wrong often enough that accented characters, quotes, and emoji turn into mojibake for some visitors and not others.
Lighthouse best practices score
Lighthouse's best practices audit covers what makes a page trustworthy to a browser: HTTPS everywhere, no deprecated APIs, no errors in the console, images shown at their real aspect ratio, a valid doctype and charset.
