As of 21 May 2016, clicking "edit" on the en.Wikipedia Mediawiki instance shows the text for editing, but then overlays it with a model text box saying:
"Welcome to Wikipedia
Anyone can edit, and every improvement helps.
Thank you for helping the world discover more!"
This dialogue is unlikely to help users focused on keyboard input methods (eg. those with accessibility needs/alternative input methods, or those who are already familiar with Wikipedia editing via the keyboard).
In the short-term the dialogue should be completely disabled/reverted until a more satisfactory and a11y-compliant solution can be developed.