It appears that the examined URL lacks a viewport <meta> tag within its <head> section.
Why is this important?
A viewport defines the visible area of a webpage. Its dimensions differ across devices—the viewport on a mobile device will be considerably smaller than on a desktop monitor. With a viewport <meta> tag, we can guide the browser in adjusting the page's dimensions and scale accordingly.
Without defining a viewport, mobile devices display pages at traditional desktop resolutions, scaled down to fit the mobile screen.
What does the Optimization check?
This Optimization activates when any internal URL is missing the viewport <meta> element in its <head> section.
How do you resolve this issue?
If specific pages aren't intended for use on varied devices and are meant for desktop viewing only (for instance, with a separate mobile version available), then overlooking this Optimization may not cause issues. However, for content intended for diverse device compatibility, a viewport <meta> tag must be incorporated within the document's head. Such a tag provides essential instructions to the browser concerning the management of the page's size and scaling configurations.
<meta name="viewport" content="width=device-width, initial-scale=1">