--- a/testing/marionette/doc/Testing.md
+++ b/testing/marionette/doc/Testing.md
@@ -1,111 +1,136 @@
-# Running Tests
+Running tests
+=============
 
-We verify and test Marionette in a couple of different ways.
-While for the server component XPCShell tests are used, the
-client and harness packages both use Python.
+We verify and test Marionette in a couple of different ways.  The
+server uses a combination of xpcshell unit tests and functional
+Python-based tests written with the Marionette harness and WPT; the
+client is tested with the same harness; and the Marionette test
+harness uses a collection of Python tests.
 
-For debugging test failures which are happing in CI, a one-click loaner from
-[Taskcluster](Taskcluster.html) can be used.
+Additionally, for debugging hard-to-reproduce test failures in CI,
+a one-click loaner from [Taskcluster] can be used.
+
+[TaskCluster]: Taskcluster.html
+
 
-## Setting up the tests
+Setup
+-----
 
-Marionette-based tests can be run in-tree with a locally built
-or downloaded Firefox through `mach`, as well as with out-of-tree
-tests with `marionette`.
+Marionette-based tests can be run through `mach` with a local
+checkout of _central_ and a build of Firefox, as well as out-of-tree
+with the `marionette` test harness.
 
 Running in-tree tests, `mach` will automatically manage the Python
 virtual environment in which your tests are run.  The Marionette
 client that is picked up is the one that is in-tree at
 _testing/marionette/client_.
 
 If you want to run tests from a downloaded test archive, you will
-need to download the `target.common.tests.zip` artifact as attached to
-Treeherder [build jobs] `B` for your system.  Extract that file and set up
-the Python Marionette client and harness by executing the following
-command:
+need to download the `target.common.tests.zip` artifact as attached
+to Treeherder [build jobs] `B` for your system.  Extract that file
+and set up the Python Marionette client and harness by executing
+the following command in a virtual environment:
 
-    % pip install -r config/marionette_requirements.txt
+	% pip install -r config/marionette_requirements.txt
 
 The tests can then be found under
-*marionette/tests/testing/marionette/harness/marionette_harness/tests* and
-can be executed with the command `marionette`.  It supports the same options as
-described below for `mach`.
+_marionette/tests/testing/marionette/harness/marionette_harness/tests_ and
+can be executed with the command `marionette`.  It supports the
+same options as described below for `mach`.
 
 [build jobs]: https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&filter-searchStr=build
 
 
-## XPCShell tests
+xpcshell unit tests
+-------------------
 
 Marionette has a set of [xpcshell] unit tests located in
 _testing/marionette/test*.js_.  These can be run this way:
 
-    % ./mach test testing/marionette/test_*.js
+	% ./mach test testing/marionette/test/unit
 
 Because tests are run in parallel and xpcshell itself is quite
 chatty, it can sometimes be useful to run the tests sequentially:
 
-    % ./mach test --sequential testing/marionette/test_error.js
+	% ./mach test --sequential testing/marionette/test/unit/test_error.js
 
 These unit tests run as part of the `X` jobs on Treeherder.
 
 [xpcshell]: https://developer.mozilla.org/en-US/docs/Mozilla/QA/Writing_xpcshell-based_unit_tests
 
 
-## Python functional tests
+Marionette functional tests
+---------------------------
 
 We also have a set of functional tests that make use of the Marionette
 Python client.  These start a Firefox process and tests the Marionette
-protocol input and output.  The following command will run all tests:
+protocol input and output.  The following command will run all
+tests:
 
-    % ./mach marionette-test
+	% ./mach marionette test
 
 But you can also run individual tests:
 
-    % ./mach marionette-test testing/marionette/harness/marionette_harness/tests/unit/test_navigation.py
+	% ./mach marionette test testing/marionette/harness/marionette_harness/tests/unit/test_navigation.py
 
-In case you want to run the tests with eg. a Nightly build of Firefox,
-mach let you do this with the --binary option:
+In case you want to run the tests with another Firefox binary:
 
-    % ./mach marionette-test --binary=/path/to/firefox-executable TEST
+	% ./mach marionette test --binary /path/to/firefox TEST
 
-When working on Marionette code it is often useful to surface the
-stdout from Firefox, which can be achived with the `--gecko-log` option.
-See [Debugging](Debugging.html) for usage instructions.
+When working on Marionette it is often useful to surface the stdout
+from Firefox, which can be achived using the `--gecko-log` option.
+See [Debugging] for usage instructions.
 
 As these are functional integration tests and pop up Firefox windows
-sporadically, a helpful tip is to surpress the window whilst you
+sporadically, a helpful tip is to suppress the window whilst you
 are running them by using Firefox’ [headless mode]:
 
-    % ./mach marionette-test -z TEST
+	% ./mach marionette test -z TEST
 
 `-z` is an alias for `--headless` and equivalent to setting the
-`MOZ_HEADLESS` output variable.  In addition to `MOZ_HEADLESS`
-there is also `MOZ_HEADLESS_WIDTH` and `MOZ_HEADLESS_HEIGHT` for
-controlling the dimensions of the no-op virtual display.  This is
-similar to using xvfb(1) which you may know from the X windowing system,
-but has the additional benefit of also working on macOS and Windows.
+`MOZ_HEADLESS` output variable.  In addition to `MOZ_HEADLESS` there
+is also `MOZ_HEADLESS_WIDTH` and `MOZ_HEADLESS_HEIGHT` for controlling
+the dimensions of the no-op virtual display.  This is similar to
+using xvfb(1) which you may know from the X windowing system, but
+has the additional benefit of also working on macOS and Windows.
 
 These functional tests will run as part of the `Mn` job on Treeherder.
 
 In addition to these two test types that specifically test the
 Marionette protocol, Marionette is used as the backend for the
 [geckodriver] WebDriver implementation.  It is served by a WPT test
 suite which effectively tests conformance to the W3C specification.
 
+[Debugging]: Debugging.html
 [headless mode]: https://developer.mozilla.org/en-US/Firefox/Headless_mode
 [geckodriver]: ../geckodriver/README.md
 
 
-## Python harness tests
+WPT functional tests
+--------------------
+
+Marionette is also indirectly tested through [geckodriver] with
+WPT.  WPT tests for WebDriver turn up with the `Wd` try job and can
+be run this way:
+
+	% ./mach wpt testing/web-platform/tests/webdriver/tests
+
+This command supports a `--webdriver-arg '-vv'` argument which
+enables more detailed logging, as well as `--jsdebugger` for opening
+the Browser Toolbox.
+
+
+Harness tests
+-------------
 
 The Marionette harness Python package has a set of unit tests, which
-are written by using the [pytest] framework. The following command will
-run all tests:
+are written by using the [pytest] framework. The following command
+will run all tests:
 
-    % ./mach python-test testing/marionette/
+	% ./mach python-test testing/marionette/
 
 To run a specific test specify the full path to the module:
 
-    % ./mach python-test testing/marionette/harness/marionette_harness/tests/harness_unit/test_serve.py
+	% ./mach python-test testing/marionette/harness/marionette_harness/tests/harness_unit/test_serve.py
 
 [pytest]: https://docs.pytest.org/en/latest/