There are tools available on line that will check your website's compatibility with the current world wide web. Both of these my husband favours and uses to check and keep his self built site close to a 100% SEO and HTML 5 compatibility level, loading ability, etc., to all browsers and media (except apparently in Safari on iPhone without putting a special coding in his html5) it may be being viewed on. He has made some changes that come up as errors, but gets a 99% rating usually.
One surprising thing that he discovered is that many sites that he checked out of sheer curiosity, often many were designed by paid professional web builders, had extreme errors and very very poor compatibility. Only 2 sites checked were in the 70 - 80% range for compatibility. So, before laying out any $$$ on an expensive professionally built website by someone who advertises themselves as a pro web builder, up on all the new HTML coding, SEO, etc., make sure to do some research.
W3C Mark Up Validation Service - for html compatibility and mark up errors in your code