Python

From Enaml's docs:

Enaml brings the declarative UI paradigm to Python in a seamlessly integrated fashion. The grammar of the Enaml language is a strict superset of Python. This means that any valid Python file is also a valid Enaml file, though the converse is not necessary true. The tight integration with Python means that the developer feels at home and uses standard Python syntax when expressing how their data models bind to the visual attributes of the UI.

. . .

Enaml’s declarative widgets provide a layer of abstraction on top of the widgets of a toolkit rendering library. Enaml ships with a backend based on Qt5/6 and third-party projects such as enaml-web and enaml-native provides alternative backends.

A maintainer of Enaml has just opened a brainstorm discussion on the next major development goals.

It's a project I've long admired, though rarely used, and I'd love to see it get some attention and a revamp. I think the bar these days has been raised by projects like QML and Slint, which provide a great context in which to set new goals.