Tab order in html/XHTML

Hi,

Does anyone know if there is anywhere in a specification document the
tab order of elements in a HTML/XHTML page (supposing DOM of page
after all JavaScript added page content). This seems like the kind of
thing I would expect to be implementor defined, but for a project I
just had a little idea for it would be nice if I could analyze a
single page and know what the tab order of that page should be.

Thanks,

Bryan Rasmussen