Piet has some good comments with CSS fonts, etc. That in itself makes Joomla a nice platform for those who need larger fonts sizes, especially the sites with font size tool bar, but that does not mean that Joomla is necessarily compatible with JAWS. I don't know if Joomla is or is not compatible. MY recommendation would be to have your potential client visit a demo site you have created with Joomla or one of from your portfolio and see if JAWS functions with JOOMLA. Also not, it may function differently with different templates based on the the template was coded.
JAWS for Windows (JFW) is a program that reads information from a computer display and speaks it to you through a speech synthesizer. I believe costs between $800-1200 depending on version. Here is a link with a good overview of JFW...
http://www.nanopac.com/jaws.htm (... if you already are aware of this information, please disregard... just trying to be helpful
)
Here is another option for visually impaired readers on your website, although should not be considered even close to being compatible with JFW... just a resource... VozMe!
http://vozme.com
Joomla site using VozMe I found with Google
http://extensions.joomla.org/extensions/4119/details
Hope this helps and good luck.
Cheers,
Steve Z