Guideline 6. Ensure that pages featuring new technologies transform gracefully.
Section 508, (Technical Standards) 1194.22
http://www.w3.org/TR/WCAG10/#gl-new-technologies
"Ensure that pages are accessible even when newer technologies are not supported or are turned off."
6.1 Organize documents so they may be read without style sheets.
6.2 Ensure that equivalents for dynamic content are updated when the dynamic content changes.
6.3 Ensure that pages are usable when scripts, applets, or other programmatic objects are turned off or not supported. If this is not possible, provide equivalent information on an alternative accessible page.
6.4 For scripts and applets, ensure that event handlers are input device-independent.
6.5 Ensure that dynamic content is accessible or provide an alternative presentation or page.
So here's the beef about Guideline 6. Make sure that everyone can see your page. That pretty much sums it up. You might ask, what about the guy who still uses windows 95? Should I make my website viewable to him too? The answer, is of course, yes. By using HTML to design your web site, you will ensure backwards compatability with older technology. If you are really itching to use lots of Flash, and Java applets and all of the other "goodies" that you can find to fill your site with, then go ahead. But offer your audience the option to view an HTML only version too. Yeah, they might not experience what you want them to experience, but if they're using dial up technology, then the irritating load times will all but guarantee that they will never come back to your site anyway.
Now, we like to use CSS. Tables were cool in the past, but now we use CSS for layout. The only problem with CSS, is that some older browswers don't support it. Make sure that you test your web pages by viewing them with styles disabled. This will let you see the heirarchal design of the site and you will see it through the eyes of a commodore 64 user (or something similar).
Although content developers are encouraged to use new technologies that solve problems raised by existing technologies, they should know how to make their pages still work with older browsers and people who choose to turn off features.