diff --git a/core/modules/system/system.base.css b/core/modules/system/system.base.css
index 11c9b418c75a5ce41b8dc8e3ede3d694c9e70264..5bbde172733a7a06ce5f56dce52ca0a01383bc5e 100644
--- a/core/modules/system/system.base.css
+++ b/core/modules/system/system.base.css
@@ -27,9 +27,7 @@
   cursor: default;
   white-space: pre;
 }
-input.form-autocomplete {
-  max-width: 100%;
-}
+
 /* Animated throbber */
 .js input.form-autocomplete {
   background-image: url(../../misc/throbber.gif);
@@ -176,6 +174,16 @@ tr .ajax-progress-throbber .throbber {
   width: 16em;
 }
 
+/**
+ *  Keep form elements from overflowing their containers.
+ */
+input {
+  max-width: 100%;
+  -moz-box-sizing: border-box;
+  -webkit-box-sizing: border-box;
+  box-sizing: border-box;
+}
+
 /**
  * Inline items.
  */