20 years ago Windows had .dlg resources that define layout and content of dialog windows. .dlg is just a binary representation of serialized window DOM.
16 years ago Visual Basic had .frm files that are close to what now known as QML.
Et cetera.
So there is nothing new with markup defining UI structure. What is new and quite valuable is CSS with its declarativeness and cascading.
20 years ago Windows had .dlg resources that define layout and content of dialog windows. .dlg is just a binary representation of serialized window DOM.
16 years ago Visual Basic had .frm files that are close to what now known as QML.
Et cetera.
So there is nothing new with markup defining UI structure. What is new and quite valuable is CSS with its declarativeness and cascading.