Hi Andy,
I can't help you with the javascript side of this question but what 
you could consider is using a "access gateway" for visually 
impaired viewers such as the one at http://www.accu.org/cgi-
bin/access/access
You should also take a look at the W3C's advice and specifications 
at http://www.w3c.org/TR/WAI-WEBCONTENT
Consider using style sheets to provide the layout and colour 
information for your website, as these can be disabled locally and 
the settings overridden if the user has a special need of any type. 
(The W3C has an excellent tutorial on CSS somewhere on their 
site too)
As a side note you should always try to write valid HTML markup (or XML 1.0 with HTML 4.01 if you want to be up-to-date :-) ) as bad
code confuses many automated accessibility accessories.

Cheers,
Nathan