css相对父元素 [爷爷级]

https://codepen.io/hellohy/pen/QMOyWR

 

<div class="l-container">
  <div class="container">    containercontainercontainercontainercontainercontainercontainercontainercontainercontainercontainercontainer
    <button>Hello!</button>
  </div>
</div>
<style>

.l-container {
position: relative;
border: 1px solid yellow;
width: 200px;
}
.container {
border: 1px solid red;
width: 200px;
height: 200px;
overflow: auto;
}
button {
border: 1px solid #ccc;
cursor: pointer;
display: block;
margin: auto;
position: absolute;
bottom: 20px;
right: 10px;
}

</style>

　　效果：

 

containercontainercontainercontainercontainercontainercontainercontainercontainercontainercontainercontainer Hello!