We’re using the Sizzle JavaScript library to help us select HTML elements in our suite of Selenium tests.  Sizzle provides all of the useful selectors from CSS3, like nth-child(), as well as some others not provided by CSS3, like nth().  Sounds great, right?  Well, yes… but take a look at the W3C recommendation for nth-child() CSS3 pseudo-class:

The index of the first child of an element is 1.

The Sizzle documentation, on the other hand, states:

:eq/:nth: Finds the Nth element on the page (e.g. :eq(5) finds the 6th element on the page).

W. T. F.?