The viewport <meta> tag does not have a width set
This means that the URL in question contains a viewport <meta> tag with no width set.
Why is this important?
The browser's viewport is the area of the window in which web content can be seen. The viewport varies with the device, and will be smaller on a mobile phone than on a computer screen. A <meta> viewport element gives the browser instructions on how to control the page's dimensions and scaling.
You can control the page dimensions on different devices by includings 'width=device-width', which matches the screen's width in device-independent pixels. If you don't set the width, mobile browsers will render the page at a desktop screen width, and then try to make the content look better by increasing font sizes and scaling the content to fit the screen, which means that font sizes may appear inconsistent to users, who may have to double-tap or pinch-to-zoom in order to see and interact with the content.
What does the Hint check?
This Hint will trigger for any internal URL which contains a viewport <meta> tag with no width value.
Examples that trigger this Hint:
This Hint would trigger for any URL that has a viewport <meta> tag in the <head>, with no width value:
How do you resolve this issue?
Set the viewport width equal to 'device-width', which gives the browser instructions to match the screen's width in device-independent pixels, e.g.
<meta name="viewport" content="width=device-width, initial-scale=1">