Is HTML and CSS Validation Important for SEO?
If you were a search engine, you would probably want to show pages in your results that were built with the proper protocols for the Internet.
Sites that are riddled with countless HTML and/or CSS validation errors will likely not appear as frequently in the search results as would pages that follow the proper syntax and validation protocols. Even so, the effect is probably somewhat negligible when you consider the more than 200 indicators Google uses for ranking websites.
Content Management System Effect
Sometimes we see that the CMS is responsible for the invalid HTML or CSS, and that the problem can not be solved unless the entire theme, or design is changed. This is where you would need to balance the time/cost benefit of fixing these errors, versus just leaving them until the site is fully overhauled.