What is DOCTYPE?
The doctype is simply a way to tell the browser—or any other parsers—what type of document they’re looking at. In the case of HTML files, it means the specific version and flavor of HTML. The doctype should always be the first item at the top of all your HTML files, appearing even before any spacing or carriage returns.
In the past, the doctype declaration was an ugly and hard-to-remember mess. For XHTML 1.0 Strict:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
And for HTML4 Transitional:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Now all you need is this:
Simple and to the point. You’ll notice that the “5” is conspicuously missing from the declaration. Although the current iteration of web markup is known as “HTML5”, it really is just an evolution of previous HTML standards—and future specifications will simply be a development of what we have today. Because browsers have to support all existing content on the Web, there’s no reliance on the doctype to tell them which features should be supported in a given document.