<div class="a b"> 2</div>
<p name="123">2333</p>
.a{
width: 100px;
height: 100px;
background-color: red
}
.b{
width: 100px;
height: 100px;
background-color: blue
}
.a.b{
width: 100px;
height: 100px;
background-color: yellow
}
/* 它会先去找.a的然后找.b的然后找.a.b的 */
[name]{
color: red
}