I’ll reply since I contributed that part (and that is a screenshot of my spec from 2008;).

Yes, tab order should follow read order but for interfaces, this isn’t obvious. Tab order within groups of controls is fairly obvious but when you UI has a top nav (with navigation on the left and links on the right), left nav, toolbar, page header, a grid, and filters…the answer is actually not that simple. Many designers start tab order with the top-left control of EVERY page and that’s often pretty irritating for users.