Progressive enhancement is just as important for SEO as it is for design and accessibility. PE helps make sure your site delivers the best user experience.
No, I'm not talking about running laps or playing dodgeball...although a dodgeball challenge of white-hat vs. black-hat SEOers is certainly something to get the adrenaline pumping. The PE here is progressive enhancement, the better looking sibling of graceful degradation.
Let's start with some basics...using neither of these means that visitors or search engines may come to your Web site and see "garbage" or blank pages--not a good presentation and akin to slamming the door in someone's face.
Graceful degradation might be seen as being focused more on the developer than the end user. Progressive enhancement though is the opposite, developing for the lowest-common-denominator, and then progressively building on top of that. PE has delivery of content at the forefront, with presentation and the bells and whistles as added enhancements.
So why is PE so good for SEO? Well, simply put, search engines are rather limited in their abilities. Sure, they may have patents to their names, highly complex algorithms directing them, supported by multimillion-dollar data centers and the most advanced computing technology, all backed by Ph.D.s in various flavors and more engineers than us average folk could ever imagine, but their spiders are pretty much limited to following simple HTML links from page to page.
Without progressive enhancement, many sites add lots of great interactive functionality at the expense of cutting search engines off from the rich content on the pages. Maybe the best way to really understand this is through a live example.
REI, one of Netconcepts clients, is probably familiar to most readers. With stores all across the country and an extensive Web presence, most of us can't help but think of REI whenever we are in need of new outdoor gear or clothing. Online retailers face some of the most challenging SEO issues. Major e-commerce sites often contain hundreds of thousands of product pages below layer upon layer of category pages. Unfortunately, more times than not, these product pages, the ideal pages for serving up highly targeted, keyword-rich content to search engines, are content lean, with little more than a few product bullet points.