Browser modes have been provided since IE8 to help developers fix issues quickly by telling a site to render like a previous version of the application:

The modes were intended to help companies continue to use legacy web applications while developers made fixes.

Despite browser modes being a poor substitute for old IEs, many find them convenient for quick and dirty testing. Starting up a virtual machine may only take a few seconds on a fast PC, but browser modes are almost instantaneous.

The following image is the browser mode menu on Windows 8.1 on Internet Explorer 11:

If you are using IE8, IE9, IE10, or IE11 and attempt to visit some specific website, but receive a message that your browser is incompatible or the layout is displayed in a wrong way, this is most likely because your browser is set in Compatibility mode.

This mode tells your browser to treat web sites as if it was using IE7. Since a lot of websites requires, at least, IE8 or better for display content properly, you receive the incompatibility message or worse. Fortunately this is very easy to resolve.

Internet Explorer 8

• In IE8, access the Tools menu.
• Make sure Compatibility View is turned off.
• Once off, access your website again and attempt to log in or visiting.

Internet Explorer 9

• In IE9, access the menu via the Gear icon and select Developer Tools or press F12 on your keyboard.
• In the bottom window of your web browser a new row will appear starting with "File".
• Click on Browser Mode and change it to IE9.
• Click on Document Mode and change it to IE9 standards.
• Once changed, access your website again and attempt to visit it. You may close the Developer Tools bar.

Internet Explorer 10

• In IE10, access the menu by pressing the ALT key to bring up the top menu and the go to Tools > Developer Tools, or press F12 on your keyboard.
• In the bottom window of your web browser a new row will appear starting with "File".
• Click on Browser Mode and change it to IE10 (NOT the IE 10 Compatibility View).
• Click on Document Mode and change it to IE10 standards.
• Once changed, access your website again and attempt to visit it. You may close the Developer Tools bar.

Internet Explorer 11

• In IE11, access the menu by pressing the ALT key to bring up the top menu and the go to Tools > Developer Tools, or press F12 on your keyboard.
• In the bottom window of your web browser a new section will appear starting with F12 display.
• Scroll down to locate the Emulation mode (icon with desktop screen and mobile screen).
• Set document mode to Edge (Default).

Set User agent string to Default

Note: you may wish to set the document mode and user agent string to IE10 as some websites could have features that have not been fully tested with IE11.

Display all websites in compatibility view: in some cases your browser may be set to always try to use compatibility mode.

• Open Internet Explorer.
• Press the ALT key to bring up the IE Menubar.
• Click on the Tools menu.
• Click on Compatibility View setting.
• Look for the checkbox "Display all the websites in compatibility view" or look to see if your website are in the list.