Quirk in general means peculiarity or characteristic. In order to maintain backward compatibility for web pages designed for older versions of some of the browsers which does not comply to standards of W3C(World Wide Web Consortium )and IETF(Internet Engineering Task Force). Its a non-standard mode.
The recent browser follow a standard mode which complies with W3C wherein the behaviour is as per HTML and CSS specifications. For HTML documents browsers use a DOCTYPE at the beginning of the document for handling the webpage in standard mode. <!DOCTYPE html> provides the purpose of activating the full standard mode.