Is it clean enough to eat off of? Is it elegant? There is a great thread going on over at HighRankings talking about the benefits of having clean HTML code on your website. The original thread starter has questions whether having clean code correlates to having high rankings in the search engines. He wants to know how you would even know if have clean code to begin with.
One the members says that having clean code and doing well in SEO is an urban myth. Its true, having sparkling clean code doesn't necessary mean you will have great search rankings nor is it a primer for ever having high rankings. Out of practice I am a big fan of cleanly coded HTML websites, its a standard for me and more out of continual principal then anything to set a good browsing environment for my visitors. It makes life a lot easier when you can easily read a website source HTML. You can diagnose errors, make changes, and reduce the size of the page for your visitors.
I think some of the confusion coming out of the clean code / SEO debate has to do with a bit of the history of SEO. Back in the days when SEO was more "optimization" than it is "link building" these days clean code did make a difference. Things were based more on on-page factors than on off page. Optimization is truely a process of cleaning up something to make it more efficient. When I optimized a website back many years ago, the primary goal was to get it cleaned up and essentially target it for selected keyphrases based on research. These days things are bit more complicated and not necessary worse either. Making sure you have clean code is really doing your due diligence for your high ranking chances so that can't hurt at all.