Cons of using Internet Explorer's Compatibility Mode

Question

What are cons of force a web site viewed in IE to compatible mode? Say we force IE9 to IE8 compatiblity mode?

• Performance drawbacks
• C

Answer 1

Using compatibility mode will NOT cause the browser to use the JavaScript engine that was present in the old version of IE.

By that I mean it will run any JavaScript code using the IE9 engine. Which was a problem for us when debugging an old product that had a problem with IE7/8.