diff --git a/mu-plugins/blocks/global-header-footer/postcss/header/menu.pcss b/mu-plugins/blocks/global-header-footer/postcss/header/menu.pcss
index ce0530261..cadd9d773 100644
--- a/mu-plugins/blocks/global-header-footer/postcss/header/menu.pcss
+++ b/mu-plugins/blocks/global-header-footer/postcss/header/menu.pcss
@@ -280,131 +280,6 @@
 	}
 }
 
-/*
-Copied Gutenberg styles to provide broader browser support.
-The code below is only applied via CSS :where selector in default Gutenberg styles.
-
-See: https://github.com/WordPress/wporg-mu-plugins/issues/159
-Source: https://github.com/WordPress/gutenberg/blob/3b2eccc289cfc90bd99252b12fc4c6e470ce4c04/packages/block-library/src/navigation/style.scss#L157
-*/
-
-.wp-block-navigation .has-child {
-	& .wp-block-navigation__submenu-container {
-		background-color: inherit;
-		color: inherit;
-		position: absolute;
-		z-index: 2;
-		display: flex;
-		flex-direction: column;
-		align-items: normal;
-
-		/* Hide until hover or focus within. */
-		opacity: 0;
-		transition: opacity 0.1s linear;
-		visibility: hidden;
-
-		/* Don't take up space when the menu is collapsed. */
-		width: 0;
-		height: 0;
-		overflow: hidden; /* Overflow is necessary to set, otherwise submenu items will take up space. */
-
-		/* Submenu items. */
-		& > .wp-block-navigation-item {
-			& > .wp-block-navigation-item__content {
-				display: flex;
-				flex-grow: 1;
-
-				/* Right-align the chevron in submenus. */
-				& .wp-block-navigation__submenu-icon {
-					margin-right: 0;
-					margin-left: auto;
-				}
-			}
-		}
-
-		/* Spacing in all submenus. */
-		& .wp-block-navigation-item__content {
-			margin: 0;
-		}
-
-		/* Submenu indentation when there's no background. */
-
-		/* Changed from source — left has no border, top needs 4px for current item border. */
-		left: 0;
-		top: calc(100% + var(--active-menu-item-border-height));
-
-		/*
-		 * Indentation for all submenus.
-		 * Nested submenus sit to the left on large breakpoints.
-		 * On smaller breakpoints, they open vertically, accordion-style.
-		 */
-
-		@media (--tablet) {
-			& .wp-block-navigation__submenu-container {
-				left: 100%;
-				top: 0;
-
-				/* Prevent the menu from disappearing when the mouse is over the gap */
-				&::before {
-					content: "";
-					position: absolute;
-					right: 100%;
-					height: 100%;
-					display: block;
-					width: 0.5em;
-					background: transparent;
-				}
-			}
-
-			/* Push inwards from right edge of submenu. */
-			& .wp-block-navigation__submenu-icon {
-				margin-right: 0.25em;
-			}
-
-			/* Reset the submenu indicator for horizontal flyouts. */
-			& .wp-block-navigation__submenu-icon svg {
-				transform: rotate(-90deg);
-			}
-		}
-	}
-
-	/*
-	 * Custom menu items.
-	 * Show submenus on hover unless they open on click.
-	 */
-
-	&:not(.open-on-click):hover > .wp-block-navigation__submenu-container {
-		visibility: visible;
-		overflow: visible;
-		opacity: 1;
-		width: auto;
-		height: auto;
-		min-width: 200px;
-	}
-
-	/* Keep submenus open when focus is within. */
-	&:not(.open-on-click):not(.open-on-hover-click):focus-within > .wp-block-navigation__submenu-container {
-		visibility: visible;
-		overflow: visible;
-		opacity: 1;
-		width: auto;
-		height: auto;
-		min-width: 200px;
-	}
-
-	/* Show submenus on click. */
-	& .wp-block-navigation-submenu__toggle[aria-expanded="true"] ~ .wp-block-navigation__submenu-container {
-		visibility: visible;
-		overflow: visible;
-		opacity: 1;
-		width: auto;
-		height: auto;
-		min-width: 200px;
-	}
-}
-
-/* End :where fix */
-
 /* Gutenberg bug: Close button is not visible in Safari; override it locally */
 .global-header__navigation .is-menu-open {
 	overflow: visible;