Wikisource:Annotations
An annotation is a note that is made while reading any form of text. This may be as simple as underlining or highlighting passages. There are no standards in this field, but there have been some developments already (Open Annotation Data Model). WS could implement a solution like hypothes.is, Textus, or develop its own.
Related to this there is a GsoC proposal by User:Rjan that aims to integrate the OKFN Annotator as a Wikimedia extension: http://annotator.wmflabs.org/wiki/Main_Page
If successful, it could be used as a basis for the Wikisource Annotation system. Afterwards, annotations in Wikisource could be linked/referenced/transcluded in Wikipedia.