The World Wide Web Consortium (W3C) was established in 1994 to lead the Web to its full potential by developing standards to promote its evolution and assure interoperability. Validating web page compliance with these standards insures that all users have consistent and equal access to the resources of your site. If your web pages are out of compliance, then you will miss out on what's next...
Perhaps due to expedience more so than ineptitude, the importance of W3C standards compliance and validation has long been disregarded by many web designers. But with the advent of Web 3.0 and the Mobile Web, the impact of that error on website ROI increases exponentially. A core requirement of the Semantic Web is that your web pages and other content be accessible and comprehensible to both Man and Machine, which makes standards compliance mandatory. W3C-compliant web pages can be properly read and interpreted regardless of what user agent might be employed: mashups and other web apps, graphical and text browsers, text-to-speech synthesizers and translators, or site robots and search engine spiders.
May throw false flags requiring manual review.
If the W3C validators (see links above) find errors in your HTML or CSS, then it's time for you to repair or redesign your website. If that's the case, then your next step might be to run some pages from your web designer's site through the same validators. And if you find errors there, then it's time for you to retain a new web designer: