diff --git a/lib/nib/vendor.styl b/lib/nib/vendor.styl
index 099919ae..5691e9c7 100644
--- a/lib/nib/vendor.styl
+++ b/lib/nib/vendor.styl
@@ -472,7 +472,7 @@ appearance()
  */
 
 tab-size()
-  vendor('tab-size', arguments, only: moz o)
+  vendor('tab-size', arguments, only: moz o official)
 
 /*
  * Vendor "overflow-scrolling" support.
diff --git a/test/cases/importance.css b/test/cases/importance.css
index ab8cf2a3..5c089c8a 100644
--- a/test/cases/importance.css
+++ b/test/cases/importance.css
@@ -115,6 +115,7 @@
   word-break: normal !important;
   -moz-tab-size: 4 !important;
   -o-tab-size: 4 !important;
+  tab-size: 4 !important;
   -webkit-hyphens: manual !important;
   -moz-hyphens: manual !important;
   -ms-hyphens: manual !important;