Most of the differences between Quirks Mode and Strict mode are highlighted below: If you want to get more information and a deeper understanding of the subject please visit this following website:
Quirks Mode is for old rules (older Browsers) while Strict Mode is for standard rules (Newer Browsers). Most Modern Browsers support both modes for backwards compatibility reasons but older browsers only support the Quirks mode by default and moderns rules will not work on older browsers. If no DOCTYPE is specified older browsers and newer browsers will process the page in quirks mode. Here is what the DOCTYPE looks like <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The moral of the story is not to use older codes with new DOCTYPES. If your old Javascritpt menus are not working the way they are supposed to, your problem may be related to the DOCTYPE that you are using.