Section 508 Web Standards
(k) A text-only page, with equivalent information or functionality, shall be provided to make a web site comply with the provisions of these standards, when compliance cannot be accomplished in any other way. The content of the text-only page shall be updated whenever the primary page changes.
What does it mean?
A text-only web page will be created when it is the only way to make information accessible and its contents will be updated when the main version changes.
How to evaluate
The alternate page is the last resort if other ways to make the page accessible have not been possible. The text-only page has equivalent information and is as up-to-date as the main page. It is easy to find the link to the text-only page without any special technology devices.
For more information
Design Resources