Making sites accessible for disabled users has always been an afterthought. Right now, making sites usable for low-vision and blind users is a cobbled together assortment of best practices and hacks. Image alt tags, semantic markup, and "skip to content" links are a few of the techniques we use to address the problem.
The problem is that these were added after the fact--and that they were designed for the static Web. It comes with it a number of problems.
Enter WAI-ARIA
Web Accessibility Initiative’s Accessible Rich Internet Applications (WAI-ARIA) is a specification designed to provide better hooks for assistive technology (AT) devices. To Web developers, it's a set of attributes that bolt on to the HTML elements you know and love.
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Support for ARIA is strong and building, with Firefox 3, Opera, WebKit / Safari, and IE 8 actively implementing this spec. Freedom Scientific's JAWS screenreader is also building in support.
Adding ARIA attributes won't break current pages , or obsolete old browsers from your site. It only makes inaccessible widgets more accessible. It will, however, make your pages invalid XHTML or HTML. This is because those namespaces do not acknowledge these new attributes. HTML 5 is expected to include these, whenever it comes out. Nonetheless, the idea of avoiding ARIA because it doesn't validate is a weak argument.
If you're designing a site that you expect will receive traffic from visually impaired users, it makes sense to check out ARIA.