On the clock page of the xfce4-panel https://docs.xfce.org/xfce/xfce4-panel/clock the below link to Pango https://developer.gnome.org/pango/stable/PangoMarkupFormat.html returns a 404, I think the new one is either this one https://developer.gnome.org/pango/stable/pango-Markup.html or this one https://developer.gnome.org/pygtk/stable/pango-markup-language.html
Fixed, thanks!