.div div div 嵌套会先首先寻找.div的所有后代div元素的dom 然后再在此dom基础上继续寻找他的后代div dom 依次类推
a.（这样的css选择会出现大量重复，后面的dom属性会覆盖前面的属性）
b. 根元素一定要固定好 倘若写的是 div div div 第一个div会把页面里所有div遍历出来 一一作为查询根元素
c 虽然多嵌套了几层 但是可以完全解决css命名重复的问题
<!-- <!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>层级选择器</title>
<style>
body{
border: 2px solid red;
margin: 0;
padding: 0;
width:300px;
}
div{
border: 3px solid yellow;
margin: 5px;
padding: 5px;
}
div div{
background: pink;
}
div div div{
background: white;
}
</style>
</head>
<body>
<div>1
<div>2
<div>3</div>
<div><div>9</div></div>
</div>
<div>4</div>
<div>5
<div>6</div>
</div>
<div>
<div>
<div></div>
</div>
</div>
</div>
</body>
</html> -->
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>层级选择器</title>
<style>
body{
border: 2px solid red;
margin: 0;
padding: 0;
width:300px;
}
div{
border: 3px solid yellow;
margin: 5px;
padding: 5px;
}
.div div div{
background: pink;
}
.div > div > div{
background: white;
}
</style>
</head>
<body>
<div class="div">1
<div>2
<div>3</div>
<div><div>9</div></div>
</div>
<div>4</div>
<div>5
<div>6</div>
</div>
<div>
<div>
<div></div>
</div>
</div>
</div>
</body>
</html>