According to HTML standards, every HTML document begins with a DOCTYPE declaration that
specifies that version of HTML the document uses. Originally, the DOCTYPE declaration was
used solely by SGML-based tools like HTML validators, that required to see that version of
HTML a document used (or claimed to use).
Today, several browsers use the document’s DOCTYPE declaration to see whether or not to
use a stricter, additional standards-oriented layout mode, or to use a “quirks” layout mode that
makes an attempt to emulate older, buggy browsers.