JSFiddle - React, Tailwind, and code Playground

HTML

<div id = "a">
    <div id = "b"></div>    
</div>

CSS

#a{
    display:inline-block;
    border:2px solid grey;
    width:100px;
    height:100px;
}
#b{
  position: relative;
  top: 50%;
  transform: translateY(-50%);
    width:20px;
    height:20px;
    border: 1px solid red;
}

JavaScript

console.log(a);