The law states:
"A text only page, with equivalent information or functionality, shall be provided to make a web site comply with the provisions of this part, when compliance cannot be accomplished in any other way. The content of the text only page shall be updated whenever the primary web page changes."
You should only create a text only web page if you have attempted to make your normal web site compliant. Most content can be made accessible and you should rarely consider making an alternative text only Web page. If you need to create a text only alternative, use LIFT (a text transcoder) to translate your web page.
The limitations of text only pages is ensuring that the content is truly equivalent and the effort it takes to maintain two different versions of the same content, i.e. keeping them in sync.