How is the DOCTYPE declaration used?
One may be led to believe that the DOCTYPE declaration at the top of the document is what tells the user agent that it is an XHTML document. However, this is not the case. The original purpose of the DOCTYPE declaration only had to do with markup validation. A validator needs to know against which document type definition (DTD) to check for compliance. Browsers don’t use validating parsers, because there is no need, so they used to ignore the DOCTYPE. When IE5/Mac was launched, it had a novelty feature: doctype switching. Its support for web standards was a major improvement compared to older version, and compared to its contemporary cousin on the Windows platform. In order to provide good standards support and still avoid breaking the millions of web sites that were written to accommodate IE’s incorrect CSS rendering, the DOCTYPE declaration was used to make an educated guess as to whether the document was ‘modern’ or ‘old-school’. This feature was then included in IE6/Win, and can no
