diff --git a/core/modules/system/css/system.module.css b/core/modules/system/css/system.module.css
index dc4a0d0..2a729c6 100644
--- a/core/modules/system/css/system.module.css
+++ b/core/modules/system/css/system.module.css
@@ -4,6 +4,19 @@
  */
 
 /**
+ * Mobile-friendly viewport.
+ *
+ * @see http://dev.w3.org/csswg/css-device-adapt/#the-viewport
+ */
+
+@-ms-viewport {
+  width: device-width;
+}
+@viewport {
+  width: device-width;
+}
+
+/**
  * Autocomplete.
  *
  * @see autocomplete.js