You may wish to begin by reading the text widget conceptual overview which gives an overview of all the objects and data types
related to the text widget and how they work together.
GtkTextTagTables as GtkBuildable
The GtkTextTagTable implementation of the GtkBuildable interface supports adding tags by specifying “tag” as the “type” attribute
of a `<child>` element.
An example of a UI definition fragment specifying tags: