By default in Safari OS X Lion you can use Tab key to navigate only on Text fields, Popup menus and use Option+Tab keys to navigate through all webpage elements, but to make navigation on all elements of the webpage bit easier you can change the Safari browser settings, follow the given steps below.
Open Safari browser
Click on Safari menu and select “Preferences”
Click on “Advanced” tab
Here under the “Universal access” section, check the “Press Tab to highlight each item on a webpage” checkbox and close the windows
Now you can use Tab key to navigate on each item of the webpage.