HTML 5 <!DOCTYPE>

HTML 5 has a lot of changes in an attempt to make it easier to develop a standard-based web page. The first thing in those changes is DOCTYPE declaration. HTML5 introduces a new DOCTYPE, which looks like this:

The declaration is placed before the tag. This tag tells the browser which HTML specification the document uses. It is an instruction to the web browser about what version of the markup language the page is written in. (It is important that you specify the DOCTYPE in all HTML documents, so that the browser knows what type of document to expect.)

This HTML5 DOCTYPE is much simpler than most of the previous DOCTYPEs, it does not require a reference to a DTD, but it recommend to still use the DOCTYPE for browsers to behave as they should. 

The good part is this new DOCTYPE is backward compatible with html4 and XHTML.  So all current browsers (IE, FF, Opera, Safari) will look at this new DOCTYPE and switch the content into standards mode - even though they don't implement HTML5.

Developers could start writing web pages using the new DOCTYPE today with HTML4-like stable featured markup while honouring things that have been clarified in HTML5.