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 hour - 8:00-8:55
2nd hour - 9:00-9:55
3rd hour - 10:00-10:55
4th hour - 11:00-12:30 Lunch
Central Time - 12:35-1:00
5th hour - 1:05-2:00
6th hour - 2:05-3:00