I think the way I would want to work with AI for web apps would be like this:

You install a fancy chrome extension or custom browser.

You go through the app on this browser. You notice something you want to change. You can then submit a prompt via this extension saying the change you want.

The trick is, the chrome extension has been following your movements through the app. This way the chrome extension can generate a lot of data the AI can use for a good prompt. I.e. The extension can get a screenshot of where you are in the app and all the pages you went through to get there. The extension can get the console logs and traces and stuff.

So with all this, the AI system gets all the material needed for a good prompt to make a good change.

In an ideal world, the AI system could then save all these details and when the change is made, guide you through the UX again. And you can check if the change was made as you wanted.

So kinda like AI flavored manual UX testing where you click around and record your findings.