<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
          
<div id='app'></div>
    

.foo {
  background: green;
}
    

ReactDOM.render(
  <div>Hello World</div>, 
  document.getElementById('app')
);