This post is about the different modes in Internet Explorer 9 and the scenarios they accommodate. “First, there’s IE9’s Standards mode is most standards-compliant, interoperable and fastest mode. It includes support for SVG, CSS3, DOM Level 3, and many other standards-based features. This is IE9’s default mode. We want site developers to use this mode as part of getting us all to goal of running “same markup”. Many real world sites still rely on legacy modes. IE9 supports Quirks mode for sites such as atlaspost.com, chase.com, and zapak.com. Lack of doctype on these sites indicates they want to render in Quirks mode. As Peter-Paul Koch points out with a set of test cases, markup that relies on Quirks mode behavior doesn’t always render as intended in other modes. To summarize, IE9 is optimized to display sites according to standards so developers can use the same markup. It’s also able to display sites in legacy modes if site developers specify that. This means users can continue to use web & web developers can move forward according to their own schedules,” notes Microsoft.