Validation is a vital part of  a professional’s workflow, helping to ensure the best cross-browser compatibility and addressing quality assurance concerns. It's the process by which you take an existing web document and compare it to the Document Type Definition (DTD) that you are declaring. For example, if you create an XHTML document using Dreamweaver tools, Dreamweaver will add the proper structural components (especially if you’re working in Dreamweaver MX 2004) required to pass conformance. However, what you add to the document might not be within a given DTD.

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