#xpath:some-xpath-expressionwas pointing, when visiting any such URL.
I didn't end up using those URLs much in practice, largely because it was a bit of a hassle crafting those XPath expressions, and to some extent because they wouldn't do much for people that did not run that user script too.
At some later point in time, I seem to have realized that I didn't, and that my custom keyboard bindings script, which I mainly use to scroll search results and other (typically paginated) similarly item-oriented pages, one item at a time (via
p-- don't ask :-), knowing about XPath expressions for the stuff it was navigating, could easily offer the feature of creating those XPath bookmarks for me. 
So I apparently implemented that feature, called it
?and promptly forgot all about it. Needless to say, I still didn't use them any more than I used to before.
Today, I realized that my (forever growing) fold comments script knows a thing or two about XPath expressions to stuff in a page (visitor comments, typically) and ought to expose that to this magic bookmark creation script. Especially since it is typically bookmarking some specific comment I wanted, anyway. It at the same time dawned on me that if I just let it share this XPath expression referencing comments (that it would fold) with the
items-xpathmeta tag of the pagination microformat that the above keyboard bindings script groks, and that script, in turn, always updated the URL when scrolling to a new place in the document, my primary use case would be covered for most of the sites I frequent a lot, at least.
I was quite bemused finding that I had implemented the
?thing, and in short work updated it to do that automatically. So, at long last, setting XPath bookmarks to in-page items of opaque, permalink deprived web pages, is a breeze. I wonder how much they will end up being used. As I never even blogged about the XPath bookmark resolver script in the first place, it hasn't picked up any mentionable user base (245 installs at present time of writing). I think it is a feature it would do the web a whole lot of good, if something like it gained traction and widespread adoption.
Not all XPath expressions make good bookmarks, of course, on web pages that change over time, but very many do, and for static content especially, it's pretty much a guarantee -- even in the face of stuff like CSS layout changes, just as permalinks do.
 Zipping through template-formatted items one item at a time with some keyboard command, so your eyes do not have to shift focal point, and can focus on just the changing content (minus static template administrativia, which suddenly cognitively completely disappears from view), is a positively huge UI improvement over either of the normal practices of reading a little, scrolling a little, and reading a little more, or skimming a page, hitting page down, reorienting yourself, and repeating the procedure.
It gets even worse with paginated web pages like search results, where, in addition to that, you have to click "Next" and wait for a moment somewhere in the work flow. Fixing this problem by pushing "endless pages" (that automatically load another pageful of content once you scroll to the end of the page) onto all your visitors, is the wrong way; many will hate you for it. As for my own browsing, I sooner or later tend to end up adding a pagination microformat producer script (if a site already has those meta tags present with correct XPath expressions in them, I of course would not need to do so myself) for my unpaginate pagination microformated web pages script.
Those scripts work well in unison, so I can key down an item at a time for pages upon pages, at worst having to wait for a moment here or there while next page is loading.