And to comment on a message from IRC:
The only reason why people want a human readable UI definition is because there aren’t any good UI tools
I disagree. For me it’s usually easier to write markup than to work with WYSIWYG. It’s not only GTK, e.g. it’s same with Markdown or even LaTeX vs things like LibreOffice/various wysiwyg html editors.
Edit: err, not question, just message.