Your HTML documents should be valid, unless your developer/designer uses html5. The latter is still in draft modus, so it won't validate either as it's not the current standard recommendation.
If your developer/designer uses CSS3, your stylesheet will NOT validate because CSS3 is not yet a standard either (draft). It's progressive and most CSS3 features don't work yet. A developer/designer can, however, take advantage of some CSS3 features that are implemented in some browsers (not all). This will instantly invalidate the stylesheet, however, which isn't a big deal, really.
CSS3 is just an extension of CSS2.1, has a few more properties to choose from, but it's still CSS and CSS3 pages do not exist, only stylesheets that utilize CSS3 features along with current standards (CSS2.1 is the current standard).