xxxxxxxxxx<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>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>