High impact · weight 8 · worth up to 2.1 points of the total score
Without it mobile browsers render the page at desktop width and shrink it to fit, which fails Google's mobile usability bar and leaves text unreadable on a phone. Editor linters such as webhint (Edge Tools in VS Code) report the same problem as "A 'viewport' meta element was not specified."
Add <meta name="viewport" content="width=device-width, initial-scale=1"> to <head>.
Check a page for this, and the other 75 checks at once
HTML parses without errors
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.
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.
