XHTML compliant code. Ok you have read about that and agree that your code should at least be syntax correct to stand a chance of a high search engine ranking.
So how do you go about validating your site html code?
Well fortunately there are several web sites which will do this for you.
Check out http://validator.w3.org/ and be prepared for some surprises.
Is your code correct or does your developer need to do some rework?