tab order is the simplest means of accessing (and assessing) web

content, so a natural order would reflect what you wanted the
reader to see. Might be title, navlist (or skip nav list),
then the toc, as a list of 'places' in the page/site.
View the content and list what you'd like to see as the 'natural'
reading order.

http://www.w3.org/TR/2006/WD-WCAG20-20060427/Overview.html#contents
wcag gives the detail if needed.

HTH