Why XPointers?

Traditional URLs are simple and easy to use, but they’re also quite limited. For one thing, a URL only points at a single, complete document. More granularity than that, such as linking to the third sentence of the seventeenth paragraph in a document, requires the author of the targeted document to manually insert named anchors at the targeted location. The author of the document doing the linking can’t do this unless he or she also has write access to the document being linked to. Even if the author doing the linking can insert named anchors into the targeted document, it’s almost always inconvenient.

It would be more useful to be able to link to a particular element or group of elements on a page without having to change the document you’re linking to. For example, given a large document such as the television listings of Chapters 4 and 5, you might want to link to only one station or one show. There are several parts to this problem. The first part is addressing the individual elements. This is the part that XPointers solve. XPointers enable you to target a given element by number, name, type, or relation to other elements in the document.

The second part of the problem is the protocol by which a browser asks a web server to send only part of a document rather than the whole thing. This is an area of active research. More work is needed. XPointers do little to solve this problem, except for providing a foundation on which such systems can build. For example, the best efforts to date are the so-called byte range extensions to HTTP available in HTTP 1.1. So far, these have not achieved widespread adoption, mostly because web authors aren’t comfortable specifying a byte range in a document. Furthermore, byte ranges are extremely fragile. Trivial edits to a document, even simple reformatting, can destroy byte range links. HTTP 1.1 does allow other range units besides raw bytes (for example, XML elements), but does not require web servers or browsers to support such units.

For the moment, therefore, an XPointer can be used as an index into a complete document, the whole of which is loaded and then positioned at the location identified by the XPointer, and even this is more than most browsers can handle. In the long term, extensions to XML, XLink, HTTP, and other protocols may allow more sophisticated uses of XPointers. For example, XInclude will let you quote a remote document by using an XPointer to tell browsers where to copy the quote in the original document, rather than retyping the text of the quote. You could include cross-references inside a document that automatically update themselves as the document is revised. These uses, however, will have to wait for the development of several next-generation technologies. For now, you must be content with precisely identifying the part of a document you want to jump to when following an XLink.

source : http://www.cafeconleche.org/books/bible3/chapters/ch18.html