css之relative

relative和absolute的关系

限制作用

1. 限制left/top/right/bottom定位；（这一点大家开发肯定都很熟悉了）
2. 限制z-index层级；
3. 限制在overflow下的嚣张气焰；

对于第2点，上代码演示：

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .green {
      width: 100px;
      height: 100px;
      background-color: green;
      z-index: 3;
      position: absolute;
    }
    .red {
      width: 100px;
      height: 100px;
      background-color: red;
      z-index: 2;
      position: absolute;
    }
  </style>
</head>
<body>
  <div class="green"></div>
  <div class="red"></div>
</body>
</html>

效果如下图，此时z-index越大，层级最高

但是，若是外层套上div元素，position设为relative，且设置的是具体的数值（不包括auto，z-index为auto不会新建层叠上下文）。属性为position:relative的元素z-index的大小决定层级高低。

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .green {
      width: 100px;
      height: 100px;
      background-color: green;
      z-index: 3;
      position: absolute;
    }
    .red {
      width: 100px;
      height: 100px;
      background-color: red;
      z-index: 2;
      position: absolute;
    }
    .relative1 {
      position: relative;
      z-index: 2;
    }
    .relative2 {
      position: relative;
      z-index: 3;
    }
  </style>
</head>
<body>
  <div class="relative1">
    <div class="green"></div>
  </div>
  <div class="relative2">
    <div class="red"></div>
  </div>
</body>
</html>

relative的最小化影响原则

1. 尽量避免使用relative
2. relative最小化

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    img {
      width: 100px;
      height: 100px;
    }
  </style>
</head>
<body>
  <div>
    <div>111</div>
    <div style="position: relative;">
      <img 
        src="https://tse3-mm.cn.bing.net/th/id/OIP-C.jyWYITfTnZ3tIelmbMj_cAHaNC?w=188&h=331&c=7&o=5&dpr=1.25&pid=1.7" 
        style="position: absolute;top: 0;right: 0;"
      >
    </div>
    <div>111</div>
  </div>
</body>
</html>

以上的理解可能有偏差，欢迎大家留意指正