XML interfaces are common in toolkits, look at Guile for GTK or Qt Quick/QML. One of the ancient X window toolkits was the Athena widget set (baked in most X se

XML interfaces are common in toolkits, look at Guile for GTK or Qt Quick/QML. One of the ancient X window toolkits was the Athena widget set (baked in most X servers, https://www.x.org/releases/X11R7.7/doc/libXaw/libXaw.html https://www.x.org/releases/X11R7.7/doc/libXaw/libXaw.html). I don't know from XML interface to design Athena GUIs.