After that, I thought that a really useful simple tool would be an online Serial Terminal, such as miniterm, minicom, PuTTY, TeraTerm or other desktop serial terminal application, but online! And after some coding, it works. You can simply access one of the URLs below and access your serial port! 🙂
I took me some time to figure this, but note that due to security concerns, it will only work if served from a HTTPS server or locally from the filesystem. If the content is served from HTTP, the serial object will not be available and the button will be disabled.