IE 11 and SharePoint Detect Compatibility Mode

Strange but true:

Identifying compatibility view in IE11 without the use of Developer tools (F12)

It’s possible to parse used IE version and its mode from User Agent string. And you don’t even have to do that yourself: there’s actually a small JavaScript-library called IE-truth, that does this for you. You can find it here: https://github.com/Gavin-Paolucci-Kleinow/ie-truth

Using this library, you can implement something like this in the script editor web part in SharePoint:

<span id="browserVersion"></span>
<script src="//cdn.rawgit.com/Gavin-Paolucci-Kleinow/ie-truth/master/ie-truth.js" type="text/javascript"></script>   <script type="text/javascript">// < ![CDATA[
// < ![CDATA[
var IE = IeVersion();
var elem = document.getElementById("browserVersion");
elem.innerHTML = "Is your browser using compatibility mode: " + IE.CompatibilityMode;
// ]]>
</script>

Full Source of source script as of 201903:

function IeVersion() {
    //Set defaults
    var value = {
		IsIE: false,
		IsEdge: false,
		EdgeHtmlVersion: 0,
		TrueVersion: 0,
		ActingVersion: 0,
		CompatibilityMode: false
	};   //Try to find the Trident version number
	var trident = navigator.userAgent.match(/Trident\/(\d+)/);
	if (trident) {
		value.IsIE = true;
		//Convert from the Trident version number to the IE version number
		value.TrueVersion = parseInt(trident[1], 10) + 4;
	}   //Try to find the MSIE number
	var msie = navigator.userAgent.match(/MSIE (\d+)/);
	if (msie) {
	    value.IsIE = true;
        	//Find the IE version number from the user agent string
		value.ActingVersion = parseInt(msie[1]);
	} else {
		//Must be IE 11 in "edge" mode
		value.ActingVersion = value.TrueVersion;
	}   //If we have both a Trident and MSIE version number, see if they're different
	if (value.IsIE && value.TrueVersion > 0 && value.ActingVersion > 0) {
		//In compatibility mode if the trident number doesn't match up with the MSIE number
		value.CompatibilityMode = value.TrueVersion != value.ActingVersion;
	}   //Try to find Edge and the EdgeHTML vesion number
	var edge = navigator.userAgent.match(/Edge\/(\d+\.\d+)$/);
	if (edge)
	{
		value.IsEdge = true;
		value.EdgeHtmlVersion = edge[1];
	}
	return value;
}