Don’t forget, you can also include React and Babel via CDN:

<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.24.0/babel.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>

Those are old versions, I just pulled out of the woodwork. Spend a few minutes Googling. This would allow you to get the least possible surface area, and of course, no build step.