Doctype in HTML5

HTML5 has a very simple doctype declaration as :

<!DOCTYPE html>

The previous doctype declaration is :

<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"

So the new doctype syntax removes all the extra declartion with previous HTML 4 doctypes.
Your doctype needs to be on the first line of your HTML file. If there’s anything else before it—even a single blank line—certain browsers will treat your page as if it has no doctype at all. Without a doctype, the browser will render your page in quirks mode. This can be a very difficult error to catch. Extra whitespace usually doesn’t matter in HTML, so my eyes tend to just skip over it, but in this case it’s very important!