JSFiddle - React, Tailwind, and code Playground

HTML

<div id="Header">  

</div>
<div id="Body">

</div>

CSS

body{
  height:100vh;
  background-color:yellow;
/*   display: flex;
  flex-direction: column; */
}
#Header{
  height:3em;
  border:solid 2px black;
  background-color:red;
}

#Body{
  border:solid 2px black;
  background-color:green;
  flex-grow: 1;
  height: 100%;
}