My code is very simple. I checked all the resources, rewrote the code into different files, updated my computer, checked the css files and everything is fine and I can't see the problem.
react.html：
<html>
<head>
<link rel="stylesheet" href="react.css">
<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"><script>
</head>
<body>
<div id="root"></div>
<script src="react.js" type="text/babel"></script>
</body>
</html>
react.js：
ReactDOM.render(<h1>Hello, World!</h1>, document.querySelector("#root"))
``` tried document.getElementById("root") as well 
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
To use
@babel/standaloneyou also need to load it, otherwisetype="text/babel"won't work.