AJAXSLT is an implementation of XSL-T in JavaScript, intended for use in fat web pages, which are nowadays referred to as AJAX applications. Because XSL-T uses XPath, it is also an implementation of XPath that can be used independently of XSL-T.
Google から XMLHttpRequest で XSLT を実現する AJAXSLT がオープンソースでリリース。XSLT を利用しなくても、Ajax で DOM のかわりに XPath を使うためにも利用可能。