XPointer: Why, what, and how?

relative addressing: allows links to places with no anchors
can point to substrings in character data and to whole tree fragments (node ranges)
used by XLink to locate remote link resources

Example of an XPointer:

                                    URI
   ----------------------------------------------------------------------
  /                                                                      \
  http://www.foo.org/bar.xml#xpointer(article/section[position() &lt;= 5])
                            |         \                                / |
                            |           ------------------------------   |
                             \                   XPointer expression    /
                              \                                        /
                               ----------------------------------------
                                          fragment identifier

(points to the first five section elements in the article root element.)

In HTML, fragment identifiers may denote anchor IDs -- XPointer generalizes that.