The difficulties with testing Internet-based Collaborative Real-time Editing Tools (CRETs) come from the requirements of collaboration, real-time and distribution. This paper considers testability as a quality attribute of CRETs requirements, and explores its design and implementation to facilitate testing CRETs. Dyanmic CRETs' testability aspects include controllability, observability and traceability. In addition, Test agent based test execution framework is built to further smooth the test progress. The automatic test framework provides functionality of estimates of test case parameters, auto-editing robot, time synchronization, testing XMLHttpRequest object, and on-line verification. To provide visualization of test execution, a SVG-based timeline diagram is developed to demonstrate the motions of testing CRETs on-line. Keywords: testability, test framework, collaboration, visualization, timeline diagram, grey-box testing.