From HTML WG Wiki
[need general description of doctype switching here]
History of doctype switching in browsers
Please consider taking a modal approach: ship a browser with 2 independent rendering systems. Use the legacy system for legacy content. Use NGLayout when rendering documents authored in either HTML 4.0 Strict or XML (this will include HTML 5.0). Pay attention to the DOCTYPE.
From there, fast-forward to March 2000, when Internet Explorer 5 for the Macintosh is released, with the first implementation of doctype switching. In the same month, Mozilla adds support for it, and shortly thereafter, a subsequent Transitional DOCTYPE with URI should trigger strict layout mode bug report leads to further refinements of the switching code (in part to more closely match the behavior in IE5 for Mac).
- Activating the Right Layout Mode Using the Doctype Declaration
- quirks mode page at Wikipedia
- Mozilla's DOCTYPE sniffing
- The Opera 9 DOCTYPE Switches
- CSS Enhancements in Internet Explorer 6: The !DOCTYPE "Switch"
- Beyond DOCTYPE: Web Standards, Forward Compatibility, and IE8
- Compatibility and IE8