For Mac OS users, there is a system setting that may not allow you to tab onto several types of elements in a web page. To change this setting:
http://www.airset.com/services/permaLink/event/jhDWpMALJBBU/13238 ...
(after school for anyone who administers the MAP test)...
http://www.airset.com/services/permaLink/event/cpQXZIUpVlBL/13273 ...