For validators from the W3C you have to comply with very strict standards, because the W3C are the ones who lay down the rules in the first place!

Make sure you have a <doctype> declaration at the top of each HTML page to specify the version of HTML/XML you are using, and also ensure that you specify the language encoding used for the document (you need to look at a dedicated site such as HTML Help to find out how to do this, but once it's done the same code can be used in every page you create).