Medium impact · weight 6 · worth up to 1.6 points of the total score
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.
Rename the repeated ids so each is used once, or switch to a class where the value was never meant to be unique.
Check a page for this, and the other 75 checks at once
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.
Page uses the HTML5 doctype
A missing or legacy doctype drops the browser into quirks mode, where box sizing and layout follow rules from the 1990s. The page then differs from every modern stylesheet assumption.
