What makes a website valid?
Validating a web page is easy. To test a web page for valid markup, simply go to http://validator.w3.org, paste the address of the page you wish to check into the input area, press "Check" and wait for the results. You might be surprised by the number of professionally designed sites that claim to be W3C valid, that fail validation miserably. With so many graphic artists claiming to be professional web designers, it's not surprising that business owners are confused about what good web site design means.
If a professional web site design company won't or can't, follow the "W3C" procedure to design valid web pages for their own site, how much "professional credibility" can they have? Will they care about the quality of your site? Will your site's search engine ranking and usability really matter to them? Why would you hire a professional web site designer who doesn't play by the rules? Before you do, validate their site. You can validate the first 100 pages of any web site with the WDG HTML Validator service, another free markup validation service. There is no valid reason, that any web designers sites won't validate. Valid web pages are important for good website design and an accessible web site. If you don't know how to write a good web page, do some basic research to find a web designer who already understands how to write a valid web page.
What is W3C validation? — Does W3C valid web page markup matter?
The "World Wide Web Consortium" provides very explicit technical specifications for "publishing valid web pages". The major search engines rely on valid hypertext markup to crawl, index and rank web pages quickly. Although most don't penalize poorly written web sites very much, they probably should. Web pages that validate will usually be re-visited more often. W3C provides a free html markup validation service to assist web site developers and web site owners maintain valid web pages. If your web site doesn't validate, your web site may be completely inaccessible to people using a specific browser, or that have functional disabilities.
Using valid CSS on web pages is important
Valid cascading style sheets will allow every browser to display web pages the way they were intended to look. Separating content and style, allows the page to load faster by reducing code bloat. Since "CSS1" and "CSS2" are already valid W3C recommendations, there is a free css validator from W3C, available for your use. W3C is still working on a new set of specifications for "CSS3" that is already available and should soon become a valid recommendation. You can read about CSS3 and some of the important changes by visiting CSS3 Info Site or W3C CSS3 working group for a good explanation.
Valid web pages get good search results
Getting good search results is the goal of every business website. High quality information always gets results. However, if search engines can't understand a web page, they will not give it the ranking it deserves. A good web page is valid, so a good search engine optimizer will ensure that all your pages validate. If you simply want beautiful web pages that only your friends can find, don't worry about validation. If you need a successful web site, make sure it's valid, accessible and provides all the information customers want. That's how a web site finds new customers and becomes profitable.