Index: src/wp-admin/css/admin-menu.css
===================================================================
--- src/wp-admin/css/admin-menu.css	(revision 44504)
+++ src/wp-admin/css/admin-menu.css	(working copy)
@@ -330,7 +330,6 @@
 #adminmenu .wp-submenu li {
 	padding: 0;
 	margin: 0;
-	overflow: hidden;
 }
 
 #adminmenu .wp-menu-image img {
@@ -547,16 +546,20 @@
 	padding: 0;
 	position: relative;
 	overflow: visible;
-	line-height: 34px;
 	background: none;
 	color: #aaa;
 	cursor: pointer;
-	outline: 0;
 }
 
-#collapse-button:hover,
+#collapse-button:hover {
+	color: #00b9eb;
+}
+
 #collapse-button:focus {
 	color: #00b9eb;
+	/* Only visible in Windows High Contrast mode */
+	outline: 1px solid transparent;
+	outline-offset: -1px;
 }
 
 #collapse-button .collapse-button-icon,
@@ -566,9 +569,12 @@
 	position: absolute;
 	top: 0;
 	left: 0;
-	line-height: 34px;
 }
 
+#collapse-button .collapse-button-label {
+	top: 8px;
+}
+
 #collapse-button .collapse-button-icon {
 	width: 36px;
 	height: 34px;
Index: src/wp-admin/css/common.css
===================================================================
--- src/wp-admin/css/common.css	(revision 44504)
+++ src/wp-admin/css/common.css	(working copy)
@@ -154,7 +154,9 @@
 	line-height: normal;
 	box-shadow: 0 0 2px 2px rgba(0,0,0,.6);
 	text-decoration: none;
-	outline: none;
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
+	outline-offset: -2px;
 }
 
 .hidden,
@@ -269,6 +271,8 @@
 	box-shadow:
 		0 0 0 1px #5b9dd9,
 		0 0 2px 1px rgba(30, 140, 190, .8);
+	/* Only visible in Windows High Contrast mode */
+	outline: 1px solid transparent;
 }
 
 .ie8 a:focus {
@@ -275,7 +279,13 @@
 	outline: #5b9dd9 solid 1px;
 }
 
-#adminmenu a:focus,
+#adminmenu a:focus {
+	box-shadow: none;
+	/* Only visible in Windows High Contrast mode */
+	outline: 1px solid transparent;
+	outline-offset: -1px;
+}
+
 .screen-reader-text:focus {
 	box-shadow: none;
 	outline: none;
@@ -613,7 +623,6 @@
 	line-height: normal; /* IE8-IE11 need this for buttons */
 	color: #0073aa; /* some of these controls are button elements and don't inherit from links */
 	cursor: pointer;
-	outline: 0;
 }
 
 .wrap .wp-heading-inline + .page-title-action {
@@ -635,6 +644,8 @@
 .wrap .page-title-action:focus {
 	border-color: #5b9dd9;
 	box-shadow: 0 0 2px rgba( 30, 140, 190, 0.8 );
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
 }
 
 .wrap h1.long-header {
Index: src/wp-admin/css/forms.css
===================================================================
--- src/wp-admin/css/forms.css	(revision 44504)
+++ src/wp-admin/css/forms.css	(working copy)
@@ -52,6 +52,8 @@
 textarea:focus {
 	border-color: #5b9dd9;
 	box-shadow: 0 0 2px rgba( 30, 140, 190, 0.8 );
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
 }
 
 /* rtl:ignore */
@@ -236,10 +238,12 @@
 	color: #72777c;
 }
 
-.form-invalid input, .form-invalid input:focus,
-.form-invalid select, .form-invalid select:focus {
+.form-invalid input,
+.form-invalid input:focus,
+.form-invalid select,
+.form-invalid select:focus {
 	border-color: #dc3232 !important;
- box-shadow: 0 0 2px rgba( 204, 0, 0, 0.8 );
+	box-shadow: 0 0 2px rgba( 204, 0, 0, 0.8 );
 }
 
 .form-table .form-required.form-invalid td:after {
Index: src/wp-admin/css/media.css
===================================================================
--- src/wp-admin/css/media.css	(revision 44504)
+++ src/wp-admin/css/media.css	(working copy)
@@ -310,10 +310,12 @@
 }
 
 #find-posts-close:focus {
-	outline: none;
 	box-shadow:
 		0 0 0 1px #5b9dd9,
 		0 0 2px 1px rgba(30, 140, 190, .8);
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
+	outline-offset: -2px;
 }
 
 #find-posts-close:before {
@@ -528,7 +530,9 @@
 	box-shadow:
 		inset 0 0 2px 3px #f1f1f1,
 		inset 0 0 0 7px #5b9dd9;
-	outline: none;
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
+	outline-offset: -6px;
 }
 
 .media-frame.mode-grid .selected.attachment {
@@ -681,6 +685,14 @@
 	box-shadow: none;
 }
 
+.upload-php .media-modal-close:focus,
+.edit-attachment-frame .edit-media-header .left:focus,
+.edit-attachment-frame .edit-media-header .right:focus {
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
+	outline-offset: -2px;
+}
+
 .upload-php .media-modal-close:focus .media-modal-icon:before,
 .upload-php .media-modal-close:hover .media-modal-icon:before {
 	color: #000;
@@ -1079,8 +1091,9 @@
 .image-editor .imgedit-settings .imgedit-help-toggle:focus {
 	color: #0074a2;
 	border-color: #5b9dd9;
-	outline: none;
 	box-shadow: 0 0 3px rgba( 0, 115, 170, .8 );
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
 }
 
 .form-table td.imgedit-response {
Index: src/wp-includes/css/admin-bar.css
===================================================================
--- src/wp-includes/css/admin-bar.css	(revision 44504)
+++ src/wp-includes/css/admin-bar.css	(working copy)
@@ -53,7 +53,6 @@
 #wpadminbar a:hover,
 #wpadminbar a img,
 #wpadminbar a img:hover {
-	outline: none;
 	border: none;
 	text-decoration: none;
 	background: none;
@@ -71,9 +70,13 @@
 #wpadminbar textarea,
 #wpadminbar div {
 	box-shadow: none;
-	outline: none;
 }
 
+#wpadminbar a:focus {
+	/* Inherits transparent outline only visible in Windows High Contrast mode */
+	outline-offset: -1px;
+}
+
 #wpadminbar {
 	direction: ltr;
 	color: #ccc;
Index: src/wp-includes/css/buttons.css
===================================================================
--- src/wp-includes/css/buttons.css	(revision 44504)
+++ src/wp-includes/css/buttons.css	(working copy)
@@ -92,9 +92,12 @@
 	padding: 0 36px;
 }
 
+/* Only visible in Windows High Contrast mode */
 .wp-core-ui .button:active,
 .wp-core-ui .button:focus {
-	outline: none;
+	outline: 2px solid transparent;
+	/* Reset inherited offset from Gutenberg */
+	outline-offset: 0;
 }
 
 .wp-core-ui .button.hidden {
@@ -190,7 +193,6 @@
 	border: 0;
 	border-radius: 0;
 	background: none;
-	outline: none;
 	cursor: pointer;
 	text-align: left;
 	/* Mimics the default link style in common.css */
@@ -211,6 +213,8 @@
 	box-shadow:
 		0 0 0 1px #5b9dd9,
 		0 0 2px 1px rgba(30, 140, 190, .8);
+	/* Only visible in Windows High Contrast mode */
+	outline: 1px solid transparent;
 }
 
 .wp-core-ui .button-link-delete {
@@ -280,7 +284,7 @@
 }
 
 .wp-core-ui .button.button-primary.button-hero {
- box-shadow: 0 2px 0 #006799;
+	box-shadow: 0 2px 0 #006799;
 }
 
 .wp-core-ui .button.button-primary.button-hero.active,
@@ -287,7 +291,7 @@
 .wp-core-ui .button.button-primary.button-hero.active:hover,
 .wp-core-ui .button.button-primary.button-hero.active:focus,
 .wp-core-ui .button.button-primary.button-hero:active {
- box-shadow: inset 0 3px 0 #006799;
+	box-shadow: inset 0 3px 0 #006799;
 }
 
 /* ----------------------------------------------------------------------------
Index: src/wp-includes/css/media-views.css
===================================================================
--- src/wp-includes/css/media-views.css	(revision 44504)
+++ src/wp-includes/css/media-views.css	(working copy)
@@ -48,8 +48,9 @@
 	box-shadow:
 		0 0 0 1px #5b9dd9,
 		0 0 2px 1px rgba(30, 140, 190, .8);
-	outline: none;
 	color: #124964;
+	/* Only visible in Windows High Contrast mode */
+	outline: 1px solid transparent;
 }
 
 .media-frame a.button {
@@ -195,6 +196,9 @@
 	color: #00a0d2;
 	border-color: #5b9dd9;
 	box-shadow: 0 0 3px rgba( 0, 115, 170, .8 );
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
+	outline-offset: -2px;
 }
 
 .media-modal-close span.media-modal-icon {
@@ -835,7 +839,9 @@
 	box-shadow:
 		inset 0 0 2px 3px #fff,
 		inset 0 0 0 7px #5b9dd9;
-	outline: none;
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
+	outline-offset: -6px;
 }
 
 .wp-core-ui .selected.attachment {
@@ -1011,6 +1017,11 @@
 		0 0 0 2px #0073aa;
 }
 
+.wp-core-ui .attachment.selected .check:focus {
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
+}
+
 .wp-core-ui .attachment.details .check .media-modal-icon,
 .wp-core-ui .media-frame.mode-grid .attachment.selected .check .media-modal-icon {
 	background-position: -21px 0;
@@ -1485,6 +1496,8 @@
 	box-shadow:
 		0 0 0 1px #fff,
 		0 0 2px 3px #5b9dd9;
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
 }
 
 .wp-core-ui .media-selection .selected.attachment {
@@ -1777,8 +1790,9 @@
 .wp-core-ui.media-modal .image-editor .imgedit-help-toggle:focus {
 	color: #0074a2;
 	border-color: #5b9dd9;
-	outline: none;
 	box-shadow: 0 0 3px rgba( 0, 115, 170, .8 );
+	/* Only visible in Windows High Contrast mode */
+	outline: 2px solid transparent;
 }
 
 .wp-core-ui.media-modal .imgedit-group-top .dashicons-arrow-down.imgedit-help-toggle {
