What is Quirks Mode?
Quirks Mode is a mode in an internet browser. Internet browsers go into Quirks Mode when a webpage does not have a doctype declaration or an incorrect doctype declaration. The browser enters this mode to try its best to display the webpage. Since the doctype declaration is necessary to display the page correctly, being in Quirks Mode is browser giving it its best effort to display the page. The webpage may be different looking than the website's owner's intentions. The browser chooses how to render elements and the resulting page might end up looking different.