Hi Igor,
If you're getting HTML or CSS errors, correct those errors ESPECIALLY your HTML errors...
I don't want this post to point you in the wrong direction so it's important to note the following:
Don't customize your website for search engines, customize it to provide the best user experience on all used browsers
Search Engine spiders work mainly in this direction: Google spider -> Make pages primarily for users, not for search engines. Don't deceive your users which is commonly referred to as "cloaking."
Avoid tricks intended to improve search engine rankings. A good rule of thumb is whether you'd feel comfortable explaining what you've done to a website that competes with you. Another useful test is to ask, "Does this help my users? Would I do this if search engines didn't exist?"
If these errors are related to layout, you will not be affected since most crawlers look at your site as plain text without trying to render them.
Some SEO's would tell you that CSS doesn't really matter, I say we cannot be 100% sure so to be on the safe side try to fix any non-layout related errors.
Regards,
Jean