whyWontThisWork.js

<div id="root"></div>
<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/babel-standalone@6/babel.min.js"></script>
<script src="https://unpkg.com/prop-types@15.6/prop-types.min.js"></script>

<script type="text/babel">
  let rootElement = document.getElementById("root")

  function SayHello(props) {
    return (
      <div>
        Hello {props.firstName} {props.secondName}!
      </div>
    )
  }

  SayHello.propTypes = {
    firstName: PropTypes.string.isRequired,
    secondName: PropTypes.string.isRequired
  }

  let element = <SayHello firstName={true} secondName={"something"}/>
  ReactDOM.render(element, rootElement)
</script>