Customise Smart Light Web app with widgets?

Is there an example web app which uses the widgets from tags.ui? I have an impression (maybe it is wrong) that web app from Smart Light does not use the widgets from tags.ui and to modify it one should write preact code?

It would be nice to quickly prototype with widgets, but documentation seems to be lacking an concrete example.

You’re correct, tags.ui are handled by the built-in renderer.

See https://mdash.net/docs/userguide/device-ui.md - note this doc is not yet exposed to the docs menu, cause there are some changes expected. But for now, you can use that.