Specify Internet Explorer 9 as the minimum supported version.

Gruntfile.js

@@ -260,7 +260,7 @@ module.exports = function (grunt) {
       options: {
         // TODO: disable `zeroUnits` optimization once clean-css 3.2 is released
         //    and then simplify the fix for https://github.com/twbs/bootstrap/issues/14837 accordingly
-        compatibility: 'ie8',
+        compatibility: 'ie9',
         keepSpecialComments: '*',
         sourceMap: true,
         noAdvanced: true

grunt/configBridge.json

@@ -15,7 +15,7 @@
       "Android >= 4",
       "Chrome >= 20",
       "Firefox >= 24",
-      "Explorer >= 8",
+      "Explorer >= 9",
       "iOS >= 6",
       "Opera >= 12",
       "Safari >= 6"