`viewport.exists`

Default: error
Plugin: viewport

A page has a <meta name="viewport"> tag.

Why: Without it, phones show the page at desktop width, zoomed out. Search engines rank pages that are not mobile-friendly lower.

When off: pages without a viewport pass.

<!-- ✗ Fails -->
<head>
  <title>About us</title>
</head>

<!-- ✓ Passes -->
<head>
  <meta name="viewport" content="width=device-width, initial-scale=1" />
</head>

References

MDN: The viewport meta element: The viewport tag makes a page fit mobile screens.
Google: Mobile-first indexing: Google indexes the mobile version of each page.