The DOCTYPE tag
Per HTML and XHTML standards, a DOCTYPE (short for “document type declaration”) informs the web browser which version of HTML you’re using, and should appear at the very top of every web page. DOCTYPE's are a key component of standards-compliant web pages, plus your code won’t render correctly or validate without them. We found a few useful links on everything you need to know about DOCTYPE's and why they are important — useful information that is worth a read.
Doctypes Overview - A good introduction to DOCTYPE's explaining why use them and where to put them in your HTML. W3- HTML doctype declaration - Good examples of various types of DOCTYPE's and how they are structured. How Important DOCTYPE declaration is for your HTML document - Examples of what can happen to your pages if you do not correctly declare a DOCTYPE.