The final point is where the concept of ‘modes’ comes in.
Changing the web browser’s rendering mode is referred to as doctype switching.
Contemporary web browsers (circa 2000+), have been built to render webpages with reference to World Wide Web Consortium (W3C) recommendations. However, the same web browsers are still used to view pages authored to historical and/or proprietary standards.
To enable backward-compatibility (old webpages working in new web browsers), contemporary web browsers can be directed to render webpages in different modes.
Standards-compliant mode is often referred to as strict mode, however this can be confusing as ‘strict’ is also a document definition type. (As a definition type, strict means that the definition
excludes the presentation attributes and elements the W3C expects to phase out, as support for cascading style sheets matures.) A webpage can use a transitional doctype and still be correctly described as ‘standards-compliant’.
In the support information supplied by the Microsoft Developer Network (the people behind the Internet Explorer web browser), quirks mode is referred to as ‘compatibility mode’. 
Due to backward-compatiblity issues caused by the 2007 release of Internet Explorer 7 (IE7), in January 2008 the IE Development Team announced plans for IE8 to provide meta element based method for declaring the content specification used to code a webpage. 
The proposed behaviour would enable a page author to request that IE8 render a webpage using
the best standards support IE can give, aka
living on the edge.
In March 2008, due to feedback from the developer community, the IE development team revised their approach. Instead of requiring content authors to add a meta element to switch IE8 to best standards mode, IE8 will default to best standards.
The meta element can still be used by content authors to specify that the webpage render using a legacy mode. For example if a page was originally built and tested for IE7, then the author could add meta element that would tell IE8 to render in IE7 standards mode.
doctype: Web standards, forward compatibility, and IE8 (Aaron Gustafson, A List Apart)
doctypeswitching is not the solution to forward compatibility.
IE8 will, by default, interpret web content in the most standards compliant way it can.