Question

What does <!DOCTYPE> describe other than the version of HTML being used and if excluded what are the consequences?

Answer 1

The doctype basically tells the browser that the page is compliant with HTML standards. Omitting the doctype can make certain browsers (chief among them, Internet Explorer) go haywire and fall back to a “Quirks Mode”, in which HTML elements aren’t rendered to standard.

In the Quirks mode the browsers violate contemporary Web format specifications in order to avoid “breaking” pages authored according to practices that were prevalent in the late 1990s. Different browsers implement different quirks. In Internet Explorer 6, 7, 8 and 9, the Quirks mode is effectively frozen IE 5.5. In other browsers, the Quirks mode is a handful of deviations from the Almost Standards mode.

Further reading: http://hsivonen.iki.fi/doctype/