Further information can be found at Jukka K. Korpela’s Quirks Mode features page. Browser comparison: which doctype triggers which mode? IE 5 Windows and Netscape 4: Doctype switching not possible; permanently locked in quirks mode. IE 6 Windows: CSS Enhancements in Internet IE 6 IE 5 Mac: No official support page, but Eric Meyer has summarized the differences. Mozilla: DOCTYPEs and list of quirks