Whats the point of DOCTYPE?

Question

I know that different doctypes are essentially about how compliant the html is, but what difference does it make what doctype you specify? Do browsers handle the same code differently depending on the doctype? Thanks

UPDATE - most answers mention quirks mode can be set off if no doctype is specified. But what would be the different between xhtml and html 4.01?

Answer 1

The DOCTYPE declaration is required to be compliant to SGML, the language HTML is an instance of.

The DOCTYPE declaration is used by some browsers to trigger different rendering modes.

Answer 2

From Wikipedia:

A Document Type Declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example, a webpage) with a Document Type Definition (DTD) (for example, the formal definition of a particular version of HTML). In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.

The HTML layout engines in modern web browsers perform DOCTYPE "sniffing" or "switching", wherein the DOCTYPE in a document served as text/html determines a layout mode, such as "quirks mode" or "standards mode". The text/html serialization of HTML5, which is not SGML-based, uses the DOCTYPE only for mode selection. Since web browsers are implemented with special-purpose HTML parsers, rather than general-purpose DTD-based parsers, they don't use DTDs and will never access them even if a URL is provided. The DOCTYPE is retained in HTML5 as a "mostly useless, but required" header only to trigger "standards mode" in common browsers.

I decided to quote this text because it answers your question better than I would :). It is important that the absence of a DOCTYPE will trigger "quirks mode" in certain browsers.