Many webmasters avoid validating their HTML and CSS code because they view it as limiting and a time sink. However, adhering to standards helps to ensure that everyone has access to the information on web pages. Standards compliance makes it easier for people with special needs to use the Web. Blind people use screen readers that depend on syntactically correct HTML. The visually impaired may use programs such as Zoomtext that rearrange and magnify web pages.
There are additional benefits to coding a standards compliant site. Mobile devices will be able render your website properly. Your website code will work with web browsers in the future. Search engines rank pages higher if they validate.
The World Wide Web Consortium (W3C) is an international industry consortium that develops open specifications (de facto standards) to enhance the interoperability of web-related products. More information about W3C activities can be obtained from the W3C website. The W3C has published an introduction to W3C standards and the W3C Markup Validation Service allows you to check code against these standards.