<!DOCTYPE html> <html> <head> <meta name="viewport" content="width=device-width, initial-scale=1.0"/> <style> body { background-color:lightgreen; } @media only screen and (max-width: 500px) { body { background-color:lightblue; } } </style> </head> <body> <p>Resize the browser window. When the width of this document is less than 500 pixels, the background-color is "lightblue", otherwise it is "lightgreen".</p> </body> </html>