Mac OS User Tabbing
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:
Open System Preferences and go to Keyboard:
Select the Shortcuts tab. Make sure the box for "Use keyboard navigation to move focus between controls" is checked.
Additional settings for Safari Browser.
Open Safari Preferences:
Select the Advanced tab. Make sure the box for "Press Tab to highlight each item on a webpage" is checked.
Ronnie Bost
Scott Brewer
Raymond Daugherty
Todd Hartsell
Ryan Crowder
Steven Dunagan 
Bradley Faw
Clement Fleming
Gordon Thacker 
Leigh Ann Davis 
John England 
Joel Ervin 
Brad Hinson 
Mason Padgett 
Kristi Parlier
Michael Petersen 
Vince Powell 
Jon Ramm 
Stephen Thacker
Andrew Bordeaux
David Broome
Marie Cawley 
Tasker Fleming 
Scott Fortune
Daniel Jenkins
Crystal Jones 
Kenneth McClamrock 
Andrew Pappas 
Dianna Singler
Dave Sloan
James Brown
Mark Driscoll
Darren Hartsell
Scott Barringer
Richard Dums
Justin Ridenhour
Jamie Blalock
Kathleen Brown
Eric Jackson
Eric Moorman 
Marty Paxton
Mike Porter
Jabarr Adams
Ryan Brown
Otis Flowers
Quintin Klutz 
Keith Rhoney
Steven Swayney
Jessica Wilkerson