css文件怎么写

@charset "utf-8";

选择器 {

    属性名1: 属性值1,

    属性名2: 属性值2,

    ...

}

<style>

@charset "utf-8";

body{

    margin: 0;

    background: #f9f9f9;

}

.shadow{

    background: #fff;

    box-shadow: 0 0 4px rgba(0, 0, 0, .2);

}

#nav{

    box-sizing: border-box;

    padding: 8px;

    height: 46px;

}

#app{

    margin: 8px;

    min-height: 300px;

}

#footer{

    box-sizing: border-box;

    padding: 8px;

    min-height: 46px

}

</style>

<div class="shadow" id="nav">nav</div>

<div class="shadow" id="app">app</div>

<div class="shadow" id="footer">footer</div>