DOCTYPE

DOCTYPE is short for "document type" and is used in web design to indicate the version of XHTML or HTML being used.

HTML5 does not rely on SGML, so it does not require a reference to a DTD.

The <!DOCTYPE html> declaration must be the first line of an HTML document and should be placed before the <html> tag.

<!DOCTYPE html>
<html>
    <head>
        <title></title>
    </head>
    <body>

    </body>
</html>

In HTML 4.01, the <!DOCTYPE> declaration references a DTD because HTML 4.01 is based on SGML. The DTD specifies the rules of the markup language so that browsers can correctly render the content.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

The DTD in the above example is called a Document Type Definition, which contains the rules for the document. The browser interprets the markup of your page based on the defined DTD and displays it accordingly.

To create a standards-compliant webpage, the DOCTYPE declaration is an essential component. Without a correct DOCTYPE for your XHTML, neither the markup nor the CSS will be effective.

XHTML 1.0 provides three options for DTD declarations:

  1. Transitional: This requires a very loose DTD and allows the continued use of HTML 4.01 markup (but in the syntax of XHTML). The complete code is as follows:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
  1. Strict: This requires a strict DTD and does not allow the use of any presentational markup or attributes, such as <br>. The complete code is as follows:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
  1. Frameset: This is specifically designed for pages that use frames. If your page contains frames, you should use this DTD. The complete code is as follows:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

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