This looks (at a glance) similar in many ways to Adobe's defunct MXML [1] which was the backbone of the Flex platform built on Flash.

Apparently this lives on in Apache Royale [2]

Using XML for an user interface description language is quite common, from XUL to XAML to JSX. In my experience, XML is well suited to this task.

1. https://en.wikipedia.org/wiki/MXML

2. https://apache.github.io/royale-docs/features/mxml