相对定位、绝对定位与固定定位的位置偏移演示 -

<!DOCTYPE html>
<html lang="zh-CN">
<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>相对定位与绝对定位</title>
</head>
<body>
  <ul>
    <li class="one">item1</li>
    <li class="two">item2</li>
  </ul>
  <div class="box">box</div>
</body>
</html>
<style>
  * {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }
  body {
    border: 5px solid red;
    height: 20em;
  }
  ul {
    border: 5px solid blue;
    height: 14em;
    width: 20em;
  }
  ul li {
    border: 1px solid black;
    list-style: none;
  }
  ul li:first-child {
    background-color: yellow;
  }
  ul li:first-child + * {
    background-color: violet;
  }
ul li.one {
  position: relative;
  top: 10px;
  left: 20px;
}
ul li.two {
  position: absolute;
  height: 200px;
  width: 200px;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  margin: auto auto;
}
.box {
  border: 1px solid darkgreen;
  background-color: greenyellow;
  height: 150px;
  width: 150px;
  position: fixed;
  right: 10px;
  bottom: 10px;
}
</style>

relative:在原始位置上稍偏移位置。
absolute:在附近位置发生元素偏移。
rixed:在HTML块内偏移，不受文档限制