W3 validation / checking CSS html etc

When I run a validation test on any of my sites (or indeed almost any website including www.catchthemes.com) I get errors and warnings, most of which mean nothing to me and in any case the sites load and work fine. So why all the errors? My worry is that any other web designer can go along to one of my clients and say: "look at all these code errors", which would presumably be a cause for concern for my clients.