Why does IE9 opens in Document Mode as IE7 standards?

Question

When I open a webpage in IE9 with DOCTYPE as

It opens Document Mode as IE7 standards.

I need default IE9 stan

Answer 1

There can be multiple reasons why it could be parsing the document under IE7 standard:

1. The server is sending a X-UA-Compatible header for IE7 in the HTTP response of the document. Check the server response headers using a tool like Fiddler.
2. The HTML document is setting a meta tag with the X-UA-Compatible property value for IE7.
3. The page is being detected automatically by IE for opening in "Compatibility view". Note here that by default all intranet sites are viewed in "Compatibility view" in IE. Uncheck the checkbox "Display intranet sites in Compatibility view" under Tools -> Compatibility view settings in IE. The "Display all websites in Compatibility view" should be unchecked too.
4. You used the Developer tools and explicitly set to view the page to render in "IE7 standards" mode. Note that this will only occur on a per client basis though.

Update 2016-01-28
As @Gordon pointed out in the comments below, another reason can be that the network administrator has set the site for compatibility view as a Group Policy on the network.
The only resolution in that case is to contact the network administrator to remove the site from the Group Policy. See HTML1203 here.