The community maintains a number of open-source projects including this documentation.
testtwf-website-src contains the source code of this very website which includes all documentation pertaining to the Open Web Platform testing effort.
web-platform-tests is the main test repository for the Open Web Platform. It contains all the existing test suites for Open Web Platform specifications except for those of the CSS Working Group.
WebIDLParser.js which are used to automatically generate tests from Web IDL snippets contained in specs.