From W3C Wiki
Avoid Needless Complexity
Although it is common practice for authors to include a doctype declaration at the top of a HTML document, browsers don't actually parse the referenced DTD. Doctype declarations are long and complex and generally authors have a hard time remembering how to write them out in full. An example of a typical doctype 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">`
So, instead of having users write a full doctype declaration they could simply write something like:
That declaration is easy to remember and serves the purpose of triggering standards-compliance mode in browsers.