Viewport

Checks that a viewport meta tag exists with width and initial-scale attributes.

The viewport meta tag tells devices how to correctly size the content for the width of the user's web browser. With search engines prioritizing mobile-friendly webpages, this is a very important setting to get right.

<meta name="viewport" content="width=device-width, initial-scale=1">

This test will check for the existence of the viewport meta tag. If it finds it, it will also check for the existence of the "width" and "initial-scale" attributes. No particular value for these attributes is preferred, although the values listed above are the most common settings for a responsive website.

For more information, please read the documentation on MDN Web Docs.