I have written elsewhere on the importance of validating Web Design Services web page. By validating, I mean checking to make sure that your HTML code does not have errors. Note that I’m not talking about spelling or grammar errors here. I’m referring to the underlying HTML code that allows the web browser to format your web page according to how you want it to appear.If you use a WYSIWYG web editor like Dreamweaver, Nvu or KompoZer, such code is usually generated behind the scenes by the editor as you type your text.Whether you write your HTML code by hand or use a WYSIWYG web editor, it is a good idea to always run the final code through HTML and CSS validators.While the search engines don’t care whether your HTML code is error-free, they rely on the basic correctness of the code to find out which portions of your web page to index. If your HTML code contains errors, it is possible that only portions of your web page are included in the search engine’s database.The errors, while undetectable in a web browser, may lead the search engine software to think that some of the text on your page is part of the HTML formatting information rather than your site’s content. As a result, the search engine may ignore that text, and your web page will not be shown in its results page.
I disagree about search engines not caring about valid HTML. I’m sure it will be a small part of Google’s ranking algorithm as they’re on a crusade to clean up/speed up the web.
You are right, google don’t care about if your html is valid or not. Long ago there were concept that crawler will break html and will not index all of your data, if any error in html, but that is a long ago.
Google even don’t bother if site is table based or DIV based.
If you want to see something interested, Google does not even validate:
I saw a video on this once. The reason is because they care more about page speed than whether it validates. Having your website W3C validated will have no impact on Google SERP results. However, I love having all of my webpages validate.