Originally reported at http://fab.wmflabs.org/T175 by @twkozlowski:
Nothing //ever// shows on cursor hover, and none of the popular images (avatars, Phabricator logo, edit menu buttons) include the "alt" attribute.
Actually, the pictures all seem to be added through CSS (as part of background for div, span, and similar tags), which is very bad for screen readers and similar software (IIRC).