In Internet Explorer 8 you have configured the following options:
"Display intranet websites in Compatibility View"=yes "Intranet Sites: Include all sites that bypass the proxy server"=yes
Additionally, you have a website that is accessed by Fully Qualified Domain name (FQDN) and is configured as part of the Trusted Sites zone. However, the site is rendered in Compatibility View instead of the expected IE8 Standards Mode.
The option "Display intranet websites in Compatibility View" uses the following algorithm in order to determine whether Compatibility View should be used:
Check, if the page belongs to the local Intranet Security zone.
Check, if the page would belong to the local Intranet zone, when ignoring the explicit Zone-mappings.
Check if the page is not using "localhost", 127.0.0.1 or ::1.
The second check with the exclusion of the zone mappings then takes care of the Intranet Option "Intranet Sites: Include all sites that bypass the proxy server". As the webpage is accessed directly by bypassing the Proxy-server, Compatibility View is used for that website.
Choose one of the following options to ensure the site is opened in IE8 Standards Mode:
Disable the "Display intranet websites in Compatibility View" option.
Disable the "Intranet Sites: Include all sites that bypass the proxy server" option, and configure all websites which bypass the proxy server to be in the security zone "Local Intranet" manually. Then, put the websites which should use the IE8 Standards Mode into Trusted Sites zone.
Add the HTTP-header name "X-UA-Compatible" with a value of "IE=8" to the websites that should run in Standards Mode.