A UI markup language could define forms that are "desktop-like" without involving a Turing complete language.

The paradigm of the HTML form with controls and a submit action has not been developed to its full potential, because it was basically derailed by JS.

It could have a lot more widgets, and there could be some standard way they exchange and validate data, without a full-blown client-side programming language.

For instance, say you want the user to pick a date. There could be a standard widget in HTML for doing that that could be dropped into a form, and it could be configured to communicate its value bi-directionally with some other field. (Perhaps like cells in a spreadsheet or that sort of reactive thing.) A rich set of standard widgets with flexible styling and layout, communicating the content of the data model to and from the server. Something like that.