W3Schools Tryit Editor

main.jsx

index.html

import { createRoot } from 'react-dom/client'

const myElement = <h1 className="myclass">Hello World</h1>;

createRoot(document.getElementById('root')).render(
  myElement
);

<!doctype html>
<html lang="en">
  <body>
    <div id="root"></div>
    <script type="module" src="/src/main.jsx"></script>
  </body>
</html>