“When people inside the building visit our web site, IE Compatibility mode is being forced on.  People on the internet don’t get compatibility mode.  Please fix.”

Internet Compatibility mode was created by Microsoft, for corporate customers.  It was first introduced with Internet Explorer 8.  Corporate customers predominately had websites coded for Internet Explorer 6.  Websites in the “Intranet” Security Zone get Compatibility Mode.

The solution to the customer query is to configure their webpage to specify what compatibility mode it is compatible with.  Microsoft have some guidance on this:
Defining document compatibility
Attaining IE8 Site Compatibility – Short Reference
Understanding Compatibility Modes in Internet Explorer 8
Specifying legacy document modes

If you visit a website with Internet Explorer, and press the F12 key, you’ll launch the Developer Tools screen.
IE Compatibility Mode

The webpage shown in the Developer Tools screenshot above is running in IE8 Standards mode.  The IE8 Standards mode has been forced by the X-UA-Compatible meta tag.


Some other things to be aware of.

Document Mode vs. Browser Mode
Document Mode.
Influences how the page displays in the browser,
The web server can force the document mode to what it wants.  In the example above, “IE8 Standards” mode has been forced by the X-UA-Compatible tag.
So, in essence, the Document Mode setting is “owned” by the web server.

Browser Mode
Browser Mode is simply put, is Internet Explorer telling the web server what it can display. 
In a corporate environment, placing a site into the Intranet Zone forces IE Compatibility Mode on.

Document Mode will in all (most?) cases override Browser Mode.
And this makes sense when you think about it.  The web server, and the webpage author, should know what their webpage page is designed for.

Gotta’s I’ve seen / heard of:

  • Placing the X-UA-Compatible meta tag in the HEAD section AFTER any scripts or CSS, DOES NOT WORK
  • The X-UA-Compatible meta tag MUST be in the HEAD section before all other elements except for the title element and other meta elements.
  • Having multiple X-UA-Compatible meta tags in the one page DOES NOT WORK.

Intranet site is identified as an Internet site when you use an FQDN or an IP address
When you access a local area network (LAN), an intranet share, or an intranet Web site by using an Internet Protocol (IP) address or a fully qualified domain name (FQDN), the share or Web site may be identified as in the Internet zone instead of in the Local intranet zone. For example, this behavior may occur if you access shares or Web sites with Microsoft Internet Explorer or Windows Internet Explorer, with Microsoft Windows Explorer, with a command prompt, or with a Windows-based program when you use an address in any one of the following formats:
  • \\Computer.childdomain.domain.com\Share
  •  \\\share
  •  file://