Not always considered a part of professional SEO (search engine optimization) work, but for us, this is the point at which all others fall at the first hurdle.

What is "W3C Markup Validation"?
Well, most of the pages on the World Wide Web are written in languages (such as HTML or XHTML) that allow web designers to structure the text, add multimedia content, and specify what appearance, or style, the page should have. These languages have their own specific grammar, vocabulary and syntax, and every document written with these languages are supposed to follow the exact rules. However, Just as text in a natural language can include spelling or grammar errors, documents using markup languages may (for various reasons) include errors. The process of verifying whether a document actually follows the exact rules for the language(s) it uses is called Markup Validation.
A document that passes this process is Valid Markup.
The W3C also provide seo tools to validate your code for FREE via their web site if you understand the coding languages and wish to validate your pages yourself.

Why should I validate my pages for search engine optimization?
1. You should make sure that your site can be viewed in all the major browsers.
2. You should validate your page using W3C to check for any errors in the HTML coding and fix where possible.

Does W3C compliance have an impact a sites search engine rankings?
No, not at the moment, but it is extremely important for browser compatibility and overall site usability. In the future, in order to judge a site's authority and quality, search engines such as Google may just include W3C validity in their algorithms.

If your site does not load correctly in browsers due to badly written code, then what's the point in being in the top 10 results, if your visitors are not able to browse your web sites content. Having a smooth and valid site is critical to your company’s reputation.

Do search engine spiders need W3C Valid Markup?
When indexing your pages, search engine spiders will "crawl" the pages and their content. They aren’t concerned with displaying it for a user but are reading the content. In other words, search engine spiders are looking at your code and are parsing it. Small errors in your HTML code can, however, disrupt the vastly important process of a spider parsing your page for all of it's relevant content

SEO is about paying close attention to every little detail of your site and its content