The document should contain a valid "doctype" declaration

Each document must contain a valid doctype declaration..

Example

Wrong

<html>

Right

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">