HTML5 has a very simple doctype declaration as :
The previous doctype declaration is :
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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!