Cómo hacer un cuadro de diálogo modal accesible

Imprescindible el artículo The Incredible Accessible Modal Dialog, que explica cómo hacer un cuadro de diálogo modal accesible que tenga en cuenta a los usuarios que utilizan el ordenador con teclado o a los usuarios ciegos (que también lo usan con el teclado, pero que presentan otras consideraciones propias).

La idea general de esta solución es:

• The first focusable item in the modal dialog should receive the keyboard focus.
• The window behind the modal dialog should not be allowed to be clicked on
• The modal dialog must trap the keyboard focus inside the modal dialog so the user can’t accidentally interact with the window behind the modal dialog.
  • When the user is on the last focusable item and presses Tab, the user should be taken to the first focusable item in the modal dialog.
  • When the user is on the first focusable item and presses Shift-Tab, the user should be taken to the last focusable item in the modal dialog.
• The position of the keyboard focus before the modal window opens must be saved, and the focus must be restored to this location after the modal dialog closes.