IE browser has the ability of emulating the lower versions.
For example, IE-11 can be made to display a page as IE-8. This is generally called the compatibility mode for IE.
If this is done by choice then it could be fine, but sometimes, browser engine itself decides that the current page should be opened using a lower version and falls into compatibility mode.
In this compatibility mode, IE starts using a lower version rendering engine due to which multiple features supported by modern browser might not run properly.
There is a way to force IE to use only the latest rendering engine it has. That is, if the browser version is IE-10 then IE-10 engine, or if the browser version is IE-11 then IE-11 engine. Such an enforcement can be established either by setting up an HTTP header or by using a meta tag.
The meta tag which needs to be included in the <head> part of HTML page in order to tell IE to use the latest engine is as following
<meta http-equiv=”X-UA-Compatible” content=”IE=edge”>
Here, we are setting the value of “X-UA-Compatible” to “IE=edge”. Reading this tag, IE knows that it is supposed to use the latest rendering engine available with it, and thus it will be forced not to downgraded itself to the compatibility mode and fall to some lower version. The use of this tag becomes very important when we want to ensure that the latest features used in our application run properly in IE promising a consistent behavior across browsers.