1. The DOCTYPE at dev.w3
A DOCTYPE is a required preamble. Note: DOCTYPEs are required for legacy reasons. When omitted, browsers tend to use a different rendering mode that is incompatible with some specifications. Including the DOCTYPE in a document ensures that the browser makes a best-effort attempt at following the relevant specifications.
2. HTML5 <!DOCTYPE> Declaration at W3School
The <!DOCTYPE> declaration must be the very first thing in your HTML5 document, before the <html> tag. This tag tells the browser which HTML specification the document uses. The doctype declaration is not an HTML tag; 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.
3. Dive Into Html5 - The Doctype
Microsoft came up with a novel solution. Before rendering a page, IE5/Mac looked at the “doctype,” which is typically the first line of the HTML source (even before the <html> element). Older pages (that relied on the rendering quirks of older browsers) generally didn’t have a doctype at all. IE5/Mac rendered these pages like older browsers did. In order to “activate” the new standards support, web page authors had to opt in, by supplying the right doctype before the <html> element.
4. Finally, a doctype anyone can remember – Building Web Pages With HTML 5 at WebMonkey
The first thing we need to do to create an HTML 5 document is use the new doctype. Now, if you’ve actually memorized the HTML 4 or XHTML 1.x doctypes, you’re better monkeys than us. Whenever we start a new page we have to bring up an old one and cut and paste the doctype definition over.