Now when IE8 is finally official released, (and I hope lots of people will drop IE6), there is the possibility (specially when you don’t built your website according webstandards!), that the page is not working anymore for IE8. To help developers, Microsoft created a quickfix to render your page in different browser and document modes, such as IE7 instead of IE8.

All you have to do is place the below meta-tag in the head of your HTML.
The best is to load it before any other meta or script tags!

<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />

X-UA-Compatible Content Property

• IE=7 -> Displays in IE7 Standards mode; Already supported in the IE8 Beta 1 release.
• IE=EmulateIE7 ->Display standards DOCTYPEs in IE7 Standards mode; Display quirks DOCTYPEs in Quirks mode; (Available through the IE June Security Update for IE8 Beta 1)
• IE=8 -> This will tell Windows Internet Explorer 8 to render each page using CSS 2.1 standards. Note that IE8 will also have a new Javascript Engine so some JS-tricks won’t work.

Browser modes

• Internet Explorer 7 Mode
  The user-agent will appear as an IE 7.0.
  Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; GTB5; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 1.1.4322; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022)
• Internet Explorer 8 Compatibility Mode
  The user-agent will appear as an IE 8.0.
  Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB5; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 1.1.4322; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022)

The developer toolbar (F12) of Internet Explorer 8 can control these settings and will override any other settings that have been set via the (above) metatags or the Compatibility View options. When changing a setting the browser will refresh and load the appropriate new configuration.

Note that by default IE8 will set all publicly accessible Internet sites to run in IE 8 mode and all local and intranet sites to IE 8 compatible mode.
When a website has the strict doctype then it will display the menu option: “Compat View” in the Page menu, which allows the user to change to compatibility mode. For sites that are using quirks mode it will not offer this option, but with the developer toolbar you always can.