5-css选择器

div{
    width: 200px;
    height: 300xp;
}

* {
    margin: 0;
    padding: 0;
}

.box > p {
    background-color: red;
}

#dada {
    width: 200px;
    height: 200px;
}

.box{
    width: 200px;
    height: 300px;
}
<div class="box"></div>

.box:hover {
    color: red;
}

div:before {
    content: '-dadaqianduan';
    background-color: yellow;
    color: red;
}

div:after{
    content: '-dadaqianduan';
    color: red;
}

.box, .box1, .box2 {
    width: 200px;
    height: 300px;
}