JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block">
  <div class="btn"></div>
</div>

CSS

.block{
  width:300px;
  height: 300px;
  position:relative;
  border:1px solid black;
}

.btn{
  width:50px;
  height:20px;
  position:absolute;
  bottom:-10px;
  left:40px;
  border:1px solid red;
}