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Before you can begin creating an XHTML document, or an HTML 4 document for that matter, you must choose which "flavor" of XHTML you will be using. To help you decide, consider the following questions:
Will your document use frames? If so, you must use the XHTML 1.0 Frameset DTD.
Do you intend to use CSS or another form of style sheet (such as XSL) for your presentational information? If so, you can use XHTML 1.0 Strict if you place all of your presentational information in the style sheet. Use XHTML 1.0 Transitional if you'll be mixing presentational elements, attributes, and style sheets.
Do you require the ability to use elements that have an intrinsic style component, such as the <i> tag for italics (versus the <em> tag for emphasis), and you cannot use style sheets to provide that instruction? If so, you must use XHTML 1.0 Transitional.