[cflsystems]

It is important, depends on the errors. You can leave some types of errors or warnings unresolved but try to get fixed all you can. For example an error "missing closing div" will break the site in IE, FF is smart enough sometimes to "fix" this error when showing the page. Validator will also show you missing alt tags or use of attributes or elements where you cannot use them. All this can lead to cross browser issues and maybe sometimes SE not indexing what you want or not at all because of missing attributes