Website Design Tips & Tricks
What is DOCTYPE?
DOCTYPE (also known as DTD), is short for document type declaration. DocTypes are a key component of compliant web pages and should be the very first code of an html document, before the <html> tag. See Document Type Declaration Wikipedia Article.
An HTML document with a doctype of XHTML 1.0 Transitional:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<title>Title of the document</title>
The content of the document......
Use the Right DocType
The DOCTYPE element is found at the very beginning of an HTML document, even before the <HTML> element. Many authoring programs, such as Dreamweaver, will automatically add a DOCTYPE to pages they generate. For a good explanation see this article Fix Your Site With the Right DOCTYPE! by Jeffrey Zeldman.
Choosing a DocType
From WDG Website Design Group. "According to HTML standards, each HTML document requires a document type declaration. The "DOCTYPE" begins the HTML document and tells a validator which version of HTML to use in checking the document's syntax." The article continues with commonly used examples of DOCTYPE.
Another good explanation of DOCTYPE by Eric Meyer and Apple Developer Connection on O'Reilly Web Dev Center Website.