You can have alternate .css style sheet files that you can switch
between. Making one .css file the main one, and another for printer
friendly, or vision impared friendly.
See this site: http://www.judyspooner.com
switches the .css style sheet that gets loaded, but retains that setting
by using cookies. The link called "Browser View".
and called whenever the visitor wants to change the style sheet.
You can view the HTML source, and see in theit loads in three .css pages (3 style sheets).
Search on Google for "alternate style sheets" and you'll see a lot of
information on using them ... primarily for printer-friendly sheets.