Okay, what is it that deters you from validating your documents? Is it the fear of running that first validation routine and finding hundreds of errors? Ya, I'll agree, those validation reports can be rather daunting to the uninitiated.
I was just performing a quality check on recently changed pages and WHAM! 47 errors on a page. Yikes, where did that come from. To the untrained eye this would have been enough to make me hit the back button and just avoid the whole process altogether. The site works and looks just fine, I have no idea what those 47 errors were, are, will be. ;)
I check the first error in sequence and I left out a closing </p> element. Run validation again and tada, all is fine. Yes, one closing </p> element missing and it caused a cascade of 46 other errors. When I first started validating (a long, long time ago), I remember looking at those errors and gasping. Then I realized that if you take them one by one as they are presented, you'll find that many of the following errors are a cascade of the ones up top. Did that make sense?
I know we've had topics in the past on the most common validation errors and I think it is always good to refresh memories and alert new Webmasters that there is nothing to FEAR when validating documents. It may appear to be somewhat major at first but once you start addressing the errors in sequence, you may be able to go from 300+ to under 50 with just a few corrections in your HTML. The example I provided above with the closing </p> element is a primary error that many run into. Just one or two missing and/or incorrectly nested elements and validation is a mess.
Remember, take the errors one at a time and start at the very top of the validation report. Don't jump down to the middle of the report as that won't do you any good. You have to address the errors in sequence so that you don't end up "chasing the cascade".
W3C Markup Validation Service
W3C CSS Validation Service
W3C RDF Validation Service
How many errors do you have on your home page this very moment? Let's get them corrected. No code dumps please. Just the first error in sequence and we can work from there. If you have hundreds, well, you'll probably need to follow along and work most of those out yourself. There will be many here who will most likely have upwards of 200, 300 and even 500+ errors. If you have errors in this number range, this is something that should be looked at soon as there are surely some in there that are potentially fatal in nature. I can understand a handful of common errors. But not a high number of uncommon errors, those could be trouble.