Browser Quirks Mode Activation Table

In order to deal with both old tag soup written to old browser quirks and new standards-compliant pages, Mozilla (and Mozilla-based browsers), Mac IE 5, Windows IE 6, Safari, Opera 7 and Konqueror 3.2 have two main layout modes. In one mode the layout engine attempts to render conforming pages according the CSS and HTML recommendations. In the other mode the layout engine tries to mimic old browsers. In Mozilla these modes are known as “the Standards mode” and “the Quirks mode” respectively.

This is the most definitive page on quirks mode settings I have found on the web - https://hsivonen.iki.fi/doctype/

I think it is a necessary evil sometimes and better to keep handy while debugging strange browser issues.