The Media Session specification is available here: https://wicg.github.io/mediasession
GitHub repository: https://github.com/WICG/mediasession
File an issue: https://github.com/WICG/mediasession/issues/new
The Media Session specification defines behaviors related to device or system features such as the actions to run when a software/hardware media key is used. These behaviors are not tested because they would depend on user agent specific implementations and device specific features.
These tests are not complete and only reflect the Blink tests that could be exported. If a reader find a behavior that could be tested and is not, they should feel free to file a bug.