Performance · 15 points
Mobile viewport
One check, stated three ways: what we measure, what would make it wrong, and how to confirm it yourself.
What this measures
The presence and shape of <meta name="viewport">.
Where this check can be wrong
A missing viewport is real; but we cannot see whether the page's own CSS is usable on a small screen, which is the thing users feel.
Verify it without taking our word for it
Open the page in a device-emulator at 390px, then on an actual phone. The phone is the correct answer.