As of 3.20, GtkLabel create both a "selection" and "link" nodes for theming using CSS, but no others. 

It would be useful if it also created a node for every elements found in the label's markup such as span, so apps to be able to apply GTK CSS widget theme to substrings in the label.

For example, calling:

> gtk_label_set_markup("<span class="warn">danger</span> normal")

Would create a "span" node with the class "danger" under the label's node, which then could then be themed using:

> label > span.warn { color: @warning_color }