The Markup Validation Service is a validator by the World Wide Web Consortium (W3C) that allows Internet users to check HTML and XHTML documents for well-formed markup. Markup validation is an important step towards ensuring the technical quality of web pages; however, is not a complete measure of Web standards conformance.
If the goal is to ensure that a client site has the best possible chance at it’s highest organic rankings, then we need to acknowledge that even just factoring in Google, there are about 200 indicators to consider. Now, I don’t know about you, but here’s the reality. With 200 factors to consider, I also have just as much of a responsibility to my clients to focus on those factors that I believe will yield the most results for the investment of time and resources.
Having hundreds of validation errors is not usually a sign of code quality, so trying to keep the number of errors, hard or soft, down is usually a good idea. Whether you really have to bring it down to zero is another discussion, as over obsessing with certain issues can be a costly thing to do. Is your client, or are you, really willing to pay to get the page to validate when all that’s not working is the fact that you use an iframe (something I’d call a soft validation error) when you’re required to use a Strict DTD? Some clients might be willing to pay for that, most I’ve worked with don’t care that much.