Always remember, the very first line of a HTML page should be the "Document Type" declaration or often known as "doctype" declaration.
Why do we need declare doctype in the first place? The standardization of HTML came a long way, it also involved the first Browser War between Internet Explorer and Netscape. In short, the standardization of HTML was chaotic, Along the line, millions of web pages were created in the web, different web page authors used different version of HTML over the decades.
As web page author, declaring doctype is to inform web browser the version of HTML that you are using, so web browsers know how to render it accordingly. If a web browser was built to comply ONLY to the latest standards of HTML, perhaps over 90% of websites in the web will appear broken.
The competition among web browsers is stiff, if a web browser does that, most web users will just abandon that web browser and it will soon be irrelevant. This is why browsers are designed to render many different version of HTML.
As web author, we should develop a habit of declaring doctype just like brushing our teeth in the morning. Without doctype, web browser will still render web page but in quirks mode.
Doctype in HTML5 is cool and simple. It is as simple as :
Unlike of different variant HTML4 or XHTML1 doctype that we are familiar with:-
Some of them can't even be fitted in one line. This is the first proof that web designer life will be easier with HTML5. A very simple HTML5 web page can be as simple as below.