OS X limits keyboard access to only text boxes and lists, this can and should be changed when testing the keyboard accessibility.
From within System Preferences -> Keyboard -> Keyboard Shortcuts tab there is a setting for allowing Full Keyboard Access. Default is "Text boxes and lists only" and this should be changed to "All Controls". Additionally, this can be enabled via keyboard shortcut: ctrl+F7
In at least some version(s) of Mac Safari, you must navigate to top-level tabs and secondary tabs via Option-Tab, rather than Tab. (And at least on the Create New page, moving between radio buttons may involve using the Right and Left Arrow keys.)
In at least some version(s) of Mac Chrome, you can navigate to top-level tabs and secondary tabs by enabling this option:
In Settings -> Show advanced settings ... -> Web Content "Pressing Tab on a webpage highlights links, as well as form fields"