Firstly, do not get paranoid when you see such errors. The good thing is that most SE Bots including Google do not do a ‘code validation check’ in order to promote rankings. This is good because 70% (or even more) of the new websites are not 100% compliant with coding standards of W3 Validator. In my personal experience I have seen hundreds of websites that are ranking pretty well with tons of W3 Validation errors and are ranked in top 3 for their queried search term.
Now coming to the main problem – Why am I seeing so many errors and how can I fix this? The problem with most new WordPress themes is that they are using CSS3 for styling the WordPress theme. This CSS3 has yet not been settled with W3 coding standards and so the CSS2 standards are used as a reference for checking the CSS and this creates error notifications. So basically all the errors you see in such a case is actually those which are coded in CSS3. The same applies to HTML5 coding which is still not a standard reference in W3 Validation. So the way to fix such reported errors would be to stick to CSS2 for now; which I believe is utterly BS.
Basically you don’t need to worry about the coding errors as long as the page is rendered as per your wishes in all the major browsers (in all their standard versions). I would recommend you to use Search Engine Spider Simulator tool at Webconfs.com (link) to check if the Search Engine spiders are seeing your site the way you want them to. If they do, you need not worry about anything. Just take a break and relax; let your SEO work for you.
Additional Tip –
- Sign up with Google Webmaster Tools and once you have validated your site with them; use the ‘Fetch as Googlebot’ option to see how Google bots see your website. If it looks fine; everything is fine.