Tabbing order goes from every tab stop within the viewlink to every tab stop in the primary document, based on sequence and tabIndex value. By default, the master element participates in the tab sequence of the primary document, even if there are no tab stops defined within the viewlink. When using a viewlink, the body element inside the viewlink is not tabbable by default; the author must set the tabIndex property if the behavior is designed to display a focus rectangle on the linked document. Setting the tabIndex property on the body of the document fragment causes the onfocus and onactivate events to fire when the viewlink document body receives focus.
Introduction to Viewlink
About Element Behaviors
Microsoft Developer Network: [Windows Internet Explorer API reference Article]