OpenMark is highly accessible.
Questions are fully keyboard enabled.
A variety of colour combinations are supported and font and image sizes may be varied using the normal browser controls of zoom in (Ctrl+) and out (Ctrl-).
In 'plain mode' questions are accessible to a screen reader. Images and equations will require the author to provide an alternative textual description ('alt text'). It is the responsibility of the question author to decide if it is possible to run a question in plain mode. For example asking a user to interpret a drawing in plain mode may require a textual description of the drawing that invalidates the question.
'Plain mode' and colour combinations are available from 'Display options' which is displayed during a test (and is not displayed on this examples site).