Many search engines rank pages based on how mobile-friendly they are. Without a viewport meta tag, mobile devices render pages at typical desktop screen widths and then scale the pages down, making them difficult to read.
Setting the viewport meta tag lets you control the width and scaling of the viewport so that it's sized correctly on all devices.
How the Lighthouse viewport meta tag audit fails
Sitefig flags pages without a viewport meta tag:
A page fails the audit unless all of these conditions are met:
The viewport meta tag contains a
contentattribute's value includes the text
Lighthouse doesn't check that
It also doesn't check for an
initial-scale key-value pair. However, you still
need to include both for your page to render correctly on mobile devices.
In the Lighthouse report UI the full PWA badge is given when you pass all of the audits in all of the PWA subcategories (Fast and reliable, Installable, and PWA optimized).
How to add a viewport meta tag
Add a viewport
<meta> tag with the appropriate key-value pairs to the
<head> of your page:
<meta name="viewport" content="width=device-width, initial-scale=1">
Here's what each key-value pair does:
width=device-widthsets the width of the viewport to the width of the device.
initial-scale=1sets the initial zoom level when the user visits the page.