JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
  <div id="menu">Menu</div>
  <div id="main">Main</div>
</div>

CSS

#container {
  width: 300px;
  height: 400px;
  border: 1px solid red;
}

#menu {
  width: 100px;
  height: 100%;
  float: left;
  border: 1px solid green;
}

#main {
  width: 100%;

  height: 100%;
  border: 1px solid blue;
}