Web Design in a Nutshell, 3rd Edition

DOCTYPE Switching

Years of lax authoring practices and techniques for dealing with inconsistent browser behaviors resulted in millions of web pages built in a way that worked, but were far from valid against the current standards. Browser developers were faced with a difficult dilemma: get rigorous about standards conformance and break nearly every web site in existence, or maintain the status quo.

When building Internet Explorer 5 for the Macintosh, development lead Tantek Çelik invented and coded a stop-gap solution that served two communities of authors: those writing standards-compliant documents and those who were authoring web documents based on familiar browser rendering behaviors.

The method now known as DOCTYPE switching uses the inclusion and content of a DOCTYPE declaration to toggle the rendering mode in certain browsers. If a modern DOCTYPE declaration is detected, it indicates that the author is standards-aware, and the browser switches into a standards-compliant rendering mode (Standards mode). If no (or if an older) declaration is detected, the browser reverts to Quirks mode. Quirks mode mimics the rendering behavior of old browsers, allowing for nonstandard code, hacks, and workarounds that are common in legacy web authoring practices. There is a third mode that some browsers implement known as Almost Standards mode that is different from true Standards mode in that it implements vertical sizing of table cells traditionally and not according to the CSS 2 specification. ...