Internet Explorer 6 DOCTYPE Bug

Microsoft sometimes use tags in ways that they shouldn't.

The DOCTYPE of a web page should have no effect on how the Javascript attached to that web page functions. Microsoft however have decreed that it should - resulting in making your Javascript code more complicated in order to work around their deliberate program bug. IE6 and earlier even ignore the DOCTYPE if it is preceded by a perfectly valid XML header so that you can't even get consistent results by including it.

Internet Explorer 6 DOCTYPE Bug