Having done some intensive research over the last couple of days to define exactly what happens when you set your IE browser to Compatibility View I thought it was worth noting a few things down.

• IE uses Compatibility View to let website developers/administrators define how a website is displayed regardless of the version of IE being used. This is because MS are moving away from their bespoke CSS implementations, used for < IE7, to the more recognised W3C version.
• The Compatibility View also affects the level of support applied to JavaScript code as functionality will change between versions of IE.
• By default, any IE user running in the Local Intranet Security zone is automatically forced into Compatibility View. However, this option can be changed either by a user with sufficient permissions or Group Policy.
• There is no way to programmatically stop users from running the site in Compatibility View. Adding the <meta> tag correctly disables the option from the Tools menu but the site can still be forced into Compatibility View either by:
  • Using the default Compatibility View settings and running in the Local Intranet security zone
  • Adding the site to the Compatibility View settings (this is a separate option to the Compatibility View switch in the Tools menu).

These are some of the useful links I have found whilst scouring the web for relevant information:

• Introducing Compatibility View
• Compatibility View and “Smart Defaults”
• Defining Document Compatibility
• IE’s Compatibility Features for Developers
• IE9’s Document Modes and JavaScript
• Testing Sites with Browser Mode vs Doc Mode
• HTTP_EQUIV Attribute
• Determining Document Mode diagram for IE9