react.html

<!doctype html>

<html lang="en">
<head>
  <meta charset="utf-8">
  <title>One file React.js</title>
</head>

<body>
    <div id="app"></div>

    <!-- Libraries: React, ReactDOM, Babel -->
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.6.1/react.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.6.1/react-dom.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.26.0/babel.min.js"></script>

    <script type="text/babel">
        class Greeting extends React.Component {
            render() {
                return (<p>{this.props.message}</p>);
            }
        }

        ReactDOM.render(
            <Greeting message='Hello World!!' />,
            document.getElementById('app')
        );
    </script>
</body>