Even though radio buttons allow selections using arrow up/down, in pages with form controls that host radio button controls between edit fields or combo boxes, keyboard navigation is murky and ill advised. User must tab to the first radio control to start navigating which becomes impossible when there are combo boxes (or edit fields) before and after the radio button control.
Suggested Use - <select...nextbind="true"</select> that binds the combo box control with the subsequent radio button control such that the next tabstop from combo box lands in the radio control. Furthermore, it informs assistive technologies of an impending radio list and allow sighted keyboard only users to tab into the radio control.