Hypertext links generally point to the top of documents and other web-connected media. Intra-document anchors exist, but these usually are established at fixed places by the author of the original document, and are not useful for others in pointing to arbitrary locations in documents they do not create.
Recently, annotation has emerged as an important new way for others to point into things and contribute independent thinking and other kinds of useful information. Central to the success and interoperability of annotation standards such as Open Annotation (openannotation.org) is the adoption of a long-term, stable approach (or possibly approaches) to anchoring annotations by selecting fragments that are robust against changes over time in the underlying document. This is referred to as intra-document anchoring.
A related, but different problem is that often documents exist in multiple formats (HTML, PDF, DOC, TXT, etc), or may be accessible in paginated or unpaginated versions. It would be helpful if an annotation created in one would also be automatically visible in the other. Also, often the same or nearly the same content (for instance with line numbers in one place, but without in another) may be available in many places (e.g. The Bible, Shakespeare, US constitution, popular song lyrics, news stories, or other examples of widely disseminated information). Should we seek ways of enabling the shared annotation of these type of documents wherever they exist? This class of problems collectively are referred to as inter-document anchoring or also content-based anchoring (aka the content addressable web).
Over the years there have been a range of suggested solutions to some or all of these problems, ranging from XPointer (XPath) to the NY Timesâ€™ Emphas