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:
1st period—Language Arts
2nd period-- Languauge Arts
3rd period—Advanced Language Arts
4th period— Language Arts
5th period—Language Arts
6th period—Language Arts
7th period—Flex
8th period—Advanced Language Arts
9th period--Conference