Offspring is a JavaScript library that applies CSS classes corresponding to some advanced CSS selectors, opening the door for more precise selectors in previously-incapable browsers.

For example, it applies "first-child" and "last-child" classes to elements which happen to be the first or last child of their parents, respectively, mimicking the :first-child (and :last-child) pseudo-classes.

The article at A List Apart goes into more detail on what the library can do and how it's used.