The button becomes active when the current page has defined a related page in its HEAD metadata. Online documentation, generated by DocBook, Docutils, GNU Texinfo, Sphinx and others, often include these reference links, and this add-on makes them usable.
This add-on requires HEAD Examiner, which scans pages on its behalf. The code of HEAD Examiner can be audited more easily, because it does not include any presentation elements itself.
