What you need to know
Mobile browsers may otherwise render a page using a wider virtual viewport and scale it down. That makes text and controls hard to use, even if the CSS layout seems to fit on desktop.
How to apply it
Include the standard width=device-width, initial-scale=1 viewport setting in the document head. Then build responsive layouts with flexible widths and test at narrow screen sizes.
Common mistake to avoid
Do not disable zoom with maximum-scale or user-scalable=no. People need pinch zoom to read content, and the page should adapt without taking that control away.