What is "quirks mode"

n. (context internet English) A technique used by some web browsers to maintain backward compatibility with older web pages that do not comply with modern standards. When such a page is detected, it is rendered in the older, non-standard way.

In computing, quirks mode refers to a technique used by some web browsers for the sake of maintaining backward compatibility with web pages designed for Internet Explorer 5 and earlier, instead of strictly complying with W3C and IETF standards in standards mode.