Client-side event logging for heterogeneous client environments

A testing model for heterogeneous client environments is enabled. A test of a computer system state transition may be specified. The test specification may include elements corresponding to test actions that cause the computer system state transition and elements corresponding to test conditions that are evaluated to generate the test results. A collection of pre-assembled executable components suitable for implementing specified tests at a wide variety of clients may be maintained, and particular test specifications may be mapped to a corresponding and optimal implementation subset of the collection. Test results may be determined based on one or more outputs of the implementation subset of executable components. A vendor and version independent browser driver may include code capable of identifying an operational set of browser capabilities among the superset of considered browser capabilities independent of vendor or version identification by a browser under test.