With a screenreader, use a moodle form with Atto in it. Verify that you can navigate to the text editor by cycling through editor fields. Verify that when you move focus to the toolbar, it reads something like "toolbar Fieldname". Verify when you move focus to the editor is says something like "textbox Fieldname". Verify when you move focus to the toolbar, you can get to an additional toolbar button "Accessibility helper". This dialogue should let you list the styles for the current selection, and give you an easier interface to list and select the links and images in the text. Verify there is a new toolbar button "Accessibility checker" verify it can check for simple accessibility errors link images with no alt text, or not enough contrast in text colors.