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:
Elementary: Kindergarten and 2nd Grade - 4:30-6:30 PM Intermediate: 4th/5th Grades - 5:00-7:00 PM Middle School: 6th-8th Grades Last Names N-Z 5:00-7:00 PM High School: 10th Grade 5:30-6:45 PM, 9th Grade 7:00-8:15 PM