testharness.js provides a framework for writing low-level tests of browser functionality in javascript. It provides a convenient API for making assertions 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 user documentation for the API is at http://web-platform-tests.org/writing-tests/testharness-api.html.
You can also read a tutorial on Using testharness.js.