Add support for testing the WebExtensions API in WPT

This RFC documents our proposal for adding support for testing the WebExtensions API in
the web platform tests.

rfcs/web_extensions.md

@@ -0,0 +1,56 @@
+# RFC #219: Add support for WebExtensions in WPT
+
+## Summary
+
+The WebExtensions API extends the capabilities of the browser. Adding support
+for WebExtensions in the web platform tests will increase interoperability
+and will help drive the standardization of this API.
+
+This RFC proposes adding a new `testharness.js` test type, `.extension.js`, to handle
+testing this API, in addition to using `testdriver.js` to load and unload extensions.
+
+##  Details
+
+Using `testdriver.js`, we added support for testing the WebExtensions API by loading
+a web extension designed to test the functionality of a specific API.
+The extension will be loaded after the tests begins, and unloaded before the
+test is finished.
+
+Most of the test execution is handled within the extension, via the 
+[browser.test](https://chromium.googlesource.com/chromium/src.git/+/master/extensions/docs/testing_api.md)
+API. We’ve elected to use these APIs since all participating browser vendors use
+`browser.test`  internally and they can easily port over existing tests to
+the web platform tests.
+
+Because these tests won’t leverage `testharness.js` directly, we’ve introduced a new
+`testharness.js`, `.extension.js`, that will create the necessary boilerplate to
+convert the `browser.test` assertions into the corresponding assertions in the test
+harness.
+
+A proposed patch is available at https://github.com/web-platform-tests/wpt/pull/50648.
+
+## Alternatives considered
+
+We considered loading the extension statically before each `testharness.js` test is run,
+but we decided against that since `testdriver.js` will allow the tests to drive the
+browser, and in the future, test functionality such as opening popups and clicking
+menu items. 
+
+We considered adding a new test type rather than using JavaScript tests, but we decided
+against it because it was a lot more work compared to using `testharness.js`.
+
+## Risks
+
+There are two potential concerns with this implementation:
+
+1. We have no precedent for tests run via a Classic command in some user agents
+   and BiDi command in others. However, the Classic implementation of loading
+   and unloading extension is modeled on the BiDi implementation, so we expect the
+   behavior to be the same. The Classic implementation is defined in the WebExtensions Community Group
+   [here](https://github.com/w3c/webextensions/blob/main/specification/webdriver-classic.bs),
+   and the BiDi implementation is defined
+   [here](https://www.w3.org/TR/webdriver-bidi/#module-webExtension).
+
+2. Another concern could be with using `browser.test` assertions and mapping them to
+   `testharness.js` assertions. With `testharness.js` not in charge of generating assertions,
+   we might end up with less useful failure messages.