I'll answer this question in two parts.
Are modal windows Accessible
The answer to what I can find seems to be Generally no. I recommend looking at this study which was done to check how screen readers react to modal windows. To quote the study
Being a screen-reader user I tested several modal dialogs, including
the jQuery modal dialog, with several screen-reader / browser
combinations. In short, the user experience was unpredictable and
ranged from the existence of the dialog not being reasonably
perceivable to the user, to not all of the content in the dialog being
perceivable to the user. It is important to note that best practices
have been developed explaining how to create an accessible modal
dialog, however it would appear that even when following these
practices modal dialogs cannot be made accessible for screen-reader
The results of the testing are given below
Results of Testing
Firefox 3.6 and JAWS 11
When activating the modal dialog nothing appeared to happen. Once I
refreshed the JAWS virtual buffer and navigated to the bottom of the
page I was able to find the dialog. The dialog was "modal" to the
extent that I could not activate links on the underlying page,
however, there was no indication that the links were disabled, they
just simply didn't work.
Firefox 3.6 and NVDA 2009.1
When activating the modal dialog NVDA announced the title of the
dialog. I was unable to navigate the content of the dialog with the
standard NVDA reading keys. When I pressed tab NVDA announced the
Close button. I was still unable to access any of the other content in
the dialog. The dialog was "modal", I was unable to interact with the
underlying page, including not being able to read any of the content
from the underlying page.
Safari 4.0.4 and VoiceOver (OS X 10.6.2)
When activating the modal dialog nothing appeared to happen. When I
navigated to the bottom of the page I found the dialog. The dialog was
"modal" to the extent that I could not activate links on the
underlying page, however, there was no indication that the links were
disabled, they just simply didn't work.
The problems identified in testing modal dialogs with screen-readers
can be broken down into two general groups. 1. Problem with perceiving
the existence of a dialog, and 2. problem with accessing the content
within the dialog, separate from the content of the underlying
page.The results of the testing are detailed below
Can Modal windows be made accessible
Yes but there are specific W3C guidelines which must be followed to make it accessible.To summarize the reference
- 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.
I recommend looking at these links for additional information on how to make a modal window or overlay accessible
The Incredible Accessible Modal Window, Version 1
Incredible Accessible Modal Window, Version 2
Is there a way to do one accessible modal? (stackoverflow)