I usually add bottle.py[0] in my project (it's single file!) and add a simple HTML form with a submit button. When I need to show a UI, I start a server, open up the URL and when the form is submitted, I shutdown the server and continue executing the script.

Using HTML means I can start off with plain HTML and no style (except user-agent) and keep building on it (change font, center it in the page, etc.). I can keep HTML, JS, CSS on the same page, add AlpineJS[1] if I need a reactive UI and so on.

The only thing missing is that I can't close the tab using JavaScript when it's finished, so the user (usually me) has to close it himself.

[0]: https://bottlepy.org/docs/dev/

[1]: https://alpinejs.dev