Building web pages with XHTML 1.1

When the user presses a key on the keyboard, that key's character is routed to a specific element. That element is said to have the focus.

When a user presses the tab key, the “focus” moves through some of the elements on the page. If you don't otherwise specify, focus moves through the hyperlinks and form elements in the order you've specified them. If the focus is currently on a hyperlink, pressing the Enter key has the same effect as clicking the mouse on the link.

You can specify that elements are traversed in a different order by the tab key using the tabindex attributes of the links and form elements. Assign a different positive number to each element's tabindex attribute, and focus will traverse those elements from the lowest-numbered to the highest, and then through all the elements that don't have a tabindex attribute.