Focus Order

Understanding WCAG2.0

A user using screen reader or the screen enlargement often moves the focus by the tab key. Movement of focus by tab key is in HTML description order if it is straightforward HTML, but can be changed by giving tabindex.When tabindex is given, care should be taken not to set the order so as to prevent understanding of the meaning.

Also, care should be taken when creating a modal dialog. For users who do not use screen readers, modal dialogs can not perform other actions until they close the modal dialog (that is, they are called "modal dialogs"). For users who use screen readers, they should behave in the same way, and there should be no such thing as moving out of the modal if you are focusing with the tab key.

Principle Operable User interface components and navigation must be operable.
Guideline Navigable Provide ways to help users navigate, find content, and determine where they are.
Criterion Focus Order
(2.4.3 A)
If the web page can navigate in sequence and its navigation order affects meaning or operation, the focusable component receives focus in an order that does not impair meaning and operability.

URL of Techniques for WCAG 2.0