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!

.l-container { position: relative; border: 1px solid rgba(255, 255, 0, 1); width: 200px }
.container { border: 1px solid rgba(255, 0, 0, 1); width: 200px; height: 200px; overflow: auto }
button { border: 1px solid rgba(204, 204, 204, 1); cursor: pointer; display: block; margin: auto; position: absolute; bottom: 20px; right: 10px }