testharness.js provides a framework for writing low-level tests of browser functionality in javascript. It provides a convenient API for making asseertions and is intended to work for both simple synchronous tests and for tests of asynchronous behaviour.
To use testharness.js you must include two scripts, in the order given:
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
Full documentation of the API is kept in the source of testharness.js.
You can also read a tutorial on Using testharness.js.