移动设备优先：

<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no">媒体标签：

@media screen and (min-width: 100px){ div{ width: 100px; height: 1000px; background: yellow; }}完整代码：

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no"><style>@media screen and (min-width: 100px){ div{ width: 100px; height: 1000px; background: yellow; }}@media screen and (min-width: 500px){ div{ width: 500px; height: 1000px; background: blue; }}@media screen and (min-width: 800px){ div{ width: 800px; height: 1000px; background: black; }}</style></head><body><div></div></body></html>效果：