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<!DOCTYPE html> <html> <head> <meta name="viewport" content="width=device-width, initial-scale=1.0"/> <meta charset="utf-8"> <title>本教程(w3big.com)</title> <style> body { background-color:lightgreen; } @media only screen and (max-width: 500px) { body { background-color:lightblue; } } </style> </head> <body> <p>重置浏览器大小,当文档的宽度小于 500 像素,背景会变为浅蓝色,否则为浅绿色。</p> </body> </html>