We built one!

https://anvil.works is the tool. It is quite explicitly based on VB and other systems like it: it's got a drag-and-drop UI designer, backed by Python. Double-click a button, and edit the Python code that runs when it's clicked. You write Python on the front end (we transpile to JS), Python on the back end, and we host it for you with a click. Check it out!

This would be super useful, but only if it is able to design using best-practices and output simple and concise CSS.

The problem with many of these tools is that the output is so convoluted that it couldn't be edited manually.

I can't tell if you're joking... Microsoft had technology like this for years and years and no one used it. (I'm not talking about Frontpage.)