Once the custom formatters feature is implemented, it needs some automated test coverage.

The tests for it should cover

• basic support (available when pref enabled, basic feature set available)
• all features (all options, all valid elements, all valid CSS, etc.)
• error handling (invalid elements in JsonML, invalid CSS, etc.)

Sebastian

As mentioned in https://phabricator.services.mozilla.com/D146389#inline-818166, the variables for testing should be logged from within the tests to provide more control and make the maintenance easier.

Sebastian