Interoperable Quirks Mode
Internet Explorer 10 provides an interoperable quirks mode that supports many features of standards mode. This enables Windows Internet Explorer to match the behavior of other leading browsers while retaining the compatibility support expected by many customers.
Document Type Declarations
Like many browsers, Internet Explorer uses the document type declaration of a webpage to determine how to display the page. If the webpage contains a standards-based declaration, such as the HTML5 document type, the page is displayed in Standards mode. If the webpage does not contain a document declaration, the page is displayed in Quirks mode.
Earlier versions of Internet Explorer would use the presence of the document type directive to determine the features available to a webpage. In order to use the latest features supported by Internet Explorer 10, you would need to add a standards-based document type, such as the HTML5 !DOCTYPE declaration shown in the following example.
Detecting Features, Not Browsers
Because the HTML5 standard requires the use of a document type directive in order to validate a webpage, it is assumed that modern websites include the HTML5 document type declaration.
In practice, this has not turned out to be the case. Many websites do not include document type directives and yet expect support for the latest features, in part because other browsers enable those features in Quirks mode. As a result, Internet Explorer 10 provides in interoperable Quirks mode for webpages that do not specify a document type.
Because many existing websites are designed to detect specific versions of Internet Explorer, Internet Explorer 10 continues to support earlier document and browser modes. Web developers are encouraged to develop standards-enabled websites that detect features, not browsers.
Internet Explorer 10 Guide for Developers