What is the difference between quirks mode and standards compliance mode, when building HTML web pages?How to tell when operating in one mode, vs. the other?  Why should people – i.e. webmasters and web designers and developers – care about choosing standards compliance mode over quirks mode?Is quirks mode ever justifiable when developing new content?
HTML and quirks vs. standards compliance modes: Explain, and tell us why we should care?
html;xhtml;standards;quirks mode
Here is probably the best article about quirks mode that I have found. I personally try to always use standard mode whenever possible since it has the best chances to work with the majority of browsers.