By default, Safari ignores the pressing of the tab button on the keyboard to follow the tab index of fields, specifically on checkbox and radio button fields.

Here is a video to show the problem: http://screencast.com/t/BsoIZ1Kz

You can fix this by going to Safari Preferences > Advanced > Press Tab…
Here is a screenshot to show you how:

And here is a video to show the result: http://screencast.com/t/Ye6yhmK7