in a browser - hitting tab tabs through all visible tabbing elements. This does not work in our modal dialogs because the tabbing goes through to the elements in the sub app below the modal area. This is a known html problem. It is necessary to resort to javascript trickery to constrain the tabbing to a specific element.
here are some notes that may be helpful:
http://stackoverflow.com/questions/11351983/how-to-limit-tab-indexes-to-just-an-overlay-and-its-elements
https://github.com/spirytoos/TABGuard/