If you look at the Internet as a group of potential consumers, the size and diversity can be boggling. The ideal goal of any web site is to have the small percentage users, who fit your customer profile, find it as easily as possible. This inevitably involves Search Engine Optimization (SEO). Search engines are constantly working to provide users with meaningful results. To accomplish this and to avoid fraud, they look for certain elements on a web page such as content, titles, and header information. Without them your page may never be found.
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In addition, just like you would not spend time reading a resume riddled with spelling and grammar errors, search engines will not spend time making sense of code that does not comply with their language standards. When you are creating a web site, or having one created for you make sure that you consider the W3C (World Wide Web Consortium) standards. Adhering to them will effect how your web site is handled by search engines.
See the W3C Page Validator for more information.