On Friday Michael Gray answered the question of whether or not W3c compliance and accessibility can impact your optimization. I agree with everything Michael said, but wanted to add a few more points.
According to Michael
From my experience having a site that is 100% code compliant doesn’t give you any SEO benefit. That said throwing up a page with complete disregard for valid code is looking for trouble.
My view exactly. Search engines are interested in your content. They want to present the most relevant results they can to a search query. If that content happens to be on a page that doesn’t validate it’s still the most relevant. If you need proof do a search and check the top results for how well they validate. My guess is that in most cases the pages won’t completely validate. You can also check the pages of the search engines themselves. Last time I looked none of them validated.
However, if your code is so poor it can have a negative impact on seo. The most obvious example is having unspiderable navigation, but it’s not the only issue. Poor code builds dams that impedes the flow of search traffic into your site. Validating your site can catch many of those errors.
Writing Valid Code Is Easier Than You Think
Does your site need to be 100% compliant? Probably not when it comes to seo, but the majority are validation errors are very easy easy to fix. Often a large number of validation errors are the result of just a few coding mistakes. I’ve found that fixing the few obvious things like forgetting to close a tag, or the lack of an alt attribute on images, cleans up most validation errors.
Contrary to what you often hear making a site 100% compliant isn’t hard either. See what’s reported as being in error and do a few searches and you can find the solution. Sure it will take a little time to find the solution and implement the fix the first time you encounter it, but once you’ve figured it out it’s easy to incorporate the next time. If you plan on developing sites you should understand why something doesn’t validate and how to correct it.
It can be difficult to validate some 3rd party applications unless you’re familiar with the code or you’re willing to put in the time to learn it, but as long as you have a desire to understand how to write compliant and accessible code there’s little excuse not to get your own code to validate.
Coding To Standards Makes Cross Browser Development Easier
As a developer I can tell you it’s much easier to code a site to standards and then tweak things for various browsers. Mostly that tweaking will be solely for Internet Explorer and if you make use of conditional comments you can even get your IE hacks to validate. Not everyone uses one browser. Remember that traffic is meaningless if it has trouble viewing your pages.
An accessible site is one that renders well in as many devices as possible and the best way to achieve that is to write valid and accessible code. I can’t tell you how many sites I’ve never gotten beyond a single page because it didn’t display well in Firefox. If you can’t be bothered to develop a site that works in the second most popular browser why should I think you’ll be bothered to resolve any issues that may come up with your products or services.
Most importantly writing accessible code is just the right thing to do.
And even if you won’t see any ranking benefit at the moment you may at some point. Accessible search has been available through Google labs since last summer and sites that validate for W3C compliance and accessibility absolutely do receive a ranking boost in it. Perhaps one day the algorithm giving that boost will find it’s way into Google’s general search engine.