add script to warn folks trying to test old IE using IE's unreliable emulation modes

docs/assets/js/ie-emulation-modes-warning.js

+// NOTICE!! DO NOT USE ANY OF THIS JAVASCRIPT
+// IT'S JUST JUNK FOR OUR DOCS!
+// ++++++++++++++++++++++++++++++++++++++++++
+/*!
+ * Copyright 2014 Twitter, Inc.
+ *
+ * Licensed under the Creative Commons Attribution 3.0 Unported License. For
+ * details, see http://creativecommons.org/licenses/by/3.0/.
+ */
+// Intended to prevent false-positive bug reports about Bootstrap not working properly in old versions of IE due to folks testing using IE's unreliable emulation modes.
+(function () {
+  'use strict';
+
+  function emulatedIEMajorVersion() {
+    var groups = /MSIE ([0-9.]+)/.exec(window.navigator.userAgent);
+    if (groups === null) {
+      return null;
+    }
+    var ieVersionNum = parseInt(groups[1], 10);
+    var ieMajorVersion = Math.floor(ieVersionNum);
+    return ieMajorVersion;
+  }
+
+  function actualNonEmulatedIEMajorVersion() {
+    // Detects the actual version of IE in use, even if it's in an older-IE emulation mode.
+    // IE JavaScript conditional compilation docs: http://msdn.microsoft.com/en-us/library/ie/121hztk3(v=vs.94).aspx
+    // @cc_on docs: http://msdn.microsoft.com/en-us/library/ie/8ka90k2e(v=vs.94).aspx
+    var jscriptVersion = new Function('/*@cc_on return @_jscript_version; @*/')(); // jshint ignore:line
+    if (jscriptVersion === undefined) {
+      return 11; // IE11+ not in emulation mode
+    }
+    if (jscriptVersion < 9) {
+      return 8; // IE8 (or lower; haven't tested on IE<8)
+    }
+    return jscriptVersion; // IE9 or IE10 in any mode, or IE11 in non-IE11 mode
+  }
+
+  var ua = window.navigator.userAgent;
+  if (ua.indexOf('Opera') > -1 || ua.indexOf('Presto') > -1) {
+    return; // Opera, which might pretend to be IE
+  }
+  var emulated = emulatedIEMajorVersion();
+  if (emulated === null) {
+    return; // Not IE
+  }
+  var nonEmulated = actualNonEmulatedIEMajorVersion();
+
+  if (emulated !== nonEmulated) {
+    window.alert('WARNING: You appear to be using IE' + nonEmulated + ' in IE' + emulated + ' emulation mode.\nIE emulation modes can behave significantly differently from ACTUAL older versions of IE.\nPLEASE DON\'T FILE BOOTSTRAP BUGS based on testing in IE emulation modes!');
+  }
+})();