Untitled

    <!DOCTYPE html>
    <html>
    <head><meta charset="utf-8"></head>
    <body>
      <script src="//fb.me/react-0.14.0.min.js"></script>
      <script src="//fb.me/react-dom-0.14.0.min.js"></script>
      <script src="//cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.25/browser.min.js"></script>
      <script type="text/babel" src="script.js"></script>
    </body>
    </html>

'use strict';
var {Component} = React;
class Hdr extends Component {
  render(){
    return (
      <header className="container">Header</header>
    );
  }
};

class App extends Component {
  render(){
    return (
      <main>
        <Hdr/>
      </main>
    );
  }
};
ReactDOM.render(<App/>, document.body);