Quick Look Preview of html files not working after iOS 13 upgrade in iPad
Apple has disabled JavaScript in Quick Look for security/privacy reasons (this was used to track users previewing local files by pinging remote servers). 

If you have your own app, use a WKWebView if you need JavaScript execution. If you are sending HTML content with JavaScript via email, send a link to a remote server instead. With a link people know they are contacting a remote server.