Low impact · weight 2 · worth up to 0.9 points of the total score
The favicon is how a user finds this tab among twenty others and how the site appears in bookmarks and history. Without a declared one the browser requests /favicon.ico and usually gets a 404.
Add <link rel="icon" href="/favicon.ico"> or an SVG equivalent to <head>.
Check a page for this, and the other 41 checks at once
Styling lives in stylesheets, not attributes
Inline style attributes cannot be cached, overridden by a stylesheet, or covered by a Content Security Policy that forbids inline styles. In quantity they also bloat every HTML response.
Viewport meta tag is present
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.
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.
