Doctypes are 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, they refer to the specific version and flavor of HTML. The doctype should always be the first item at the top of all your HTML files. 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" "https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
And for HTML4 Transitional:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "https://www.w3.org/TR/html4/loose.dtd">
Over the years, code editing software began to provide HTML templates with the doctype already included, or else they offered a way to automatically insert one. And naturally, a quick web search will easily bring up the code to insert whatever doctype you require.
Although having that long string of text at the top of our documents hasn’t really hurt us (other than forcing our sites’ viewers to download a few extra bytes), HTML5 has done away with that indecipherable eyesore. 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.
The Principles of Beautiful Web Design, 4th Edition
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