Hi again Gaia,
Julien suggested I mention it on the list so here it is.
If you are writing or maintaining Gij tests, you can now enable various accessibility checks that are now built in into marionette. You do not actually need to write anything specific to test for accessibility as the checks run during normal flow of actions and events.
For example, whenever you tests clicks or taps on an element marionette will also check if an accessibility (screen reader) user can do that as well. Whenever your test checks if something is enabled or visible, marionette will check if this is also true for a screen reader user. It also does some nice things in terms of checking for keyboard focusability, obstruction with pointer-events: none, labelling and naming, etc.
There are 2 implications:
You can also try enabling accessibility checks for your existing test howeverm if something in the app that the test checks is not accessible (for example an icon button has no label, or something is hidden off screen, but actually visible to the screen reader), you will see your tests failing. Example from  also applies.
Note: I ran most of the tests with the a11y checks in place already and enabled most of the ones that reliably pass. This means that if some tests do not yet have them enabled, they are likely failing.