diff --git a/src/wp-admin/css/customize-controls.css b/src/wp-admin/css/customize-controls.css
index 15927a63b9..5d7edd3567 100644
--- a/src/wp-admin/css/customize-controls.css
+++ b/src/wp-admin/css/customize-controls.css
@@ -1137,6 +1137,7 @@ p.customize-section-description {
 }
 #customize-theme-controls #sub-accordion-section-custom_css textarea {
 	-moz-tab-size: 4;
+	-o-tab-size: 4;
 	tab-size: 4;
 }