We changed the link documenting Pango markup from https://developer.gnome.org/pango/stable/PangoMarkupFormat.html to https://developer.gnome.org/pygtk/stable/pango-markup-language.html in r35188. That new link is now dead, so we need to fix this again.
(unfortunately there is no bug reference for that last change so the context is unknown)
Pango's official documentation is now back up and running, so let's go back to that.
Verified the problem in the old documentation  and that it's fixed now in the PDF  and in the HTML . The current link looks correct.