Low impact · weight 1 · worth up to 0.3 points of the total score
A page opened with target="_blank" can reach back to the page that opened it and send it elsewhere. Current browsers assume noopener, but older ones and some in-app browsers do not, and saying it costs nothing.
Add rel="noopener" (or noreferrer) to every link with target="_blank".
Check a page for this, and the other 70 checks at once
Email addresses are not left for spam bots
Bots crawl the web collecting every address written in a page or a mailto: link, and those mailboxes fill with spam within weeks. A contact form, or an address the page assembles with a little JavaScript, keeps the inbox usable.
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.
