diff --git a/wagtail/wagtailadmin/static_src/wagtailadmin/scss/components/_main-nav.scss b/wagtail/wagtailadmin/static_src/wagtailadmin/scss/components/_main-nav.scss
index a540966f7..46e5006d5 100644
--- a/wagtail/wagtailadmin/static_src/wagtailadmin/scss/components/_main-nav.scss
+++ b/wagtail/wagtailadmin/static_src/wagtailadmin/scss/components/_main-nav.scss
@@ -424,7 +424,7 @@ body.explorer-open {
             top: 0;
             right: 0;
             z-index: -1;
-            box-shadow: 0px 0px 2px rgba(255, 255, 255, 0.9);
+            box-shadow: 0 0 2px rgba(255, 255, 255, 0.9);
         }
 
         .nav-main {