all browsers needed two modes: quirks mode for the old rules, strict mode for the standard. IE Mac was the first browser to implement the two modes, and IE Windows 6, Mozilla, Safari, and Opera followed suit. IE 5 Windows, as well as older browsers like Netscape 4, are permanently locked in quirks mode.
Choosing which mode to use requires a trigger, and this trigger was found in ’doctype switching’. According to the standards, any (X)HTML document should have a doctype which tells the world at large which flavour of (X)HTML the document is using
