Index: src/js/_enqueues/admin/postbox.js
===================================================================
--- src/js/_enqueues/admin/postbox.js	(revision 47239)
+++ src/js/_enqueues/admin/postbox.js	(working copy)
@@ -363,7 +363,7 @@
 			if ( side.length ) {
 				if ( side.children('.postbox:visible').length )
 					side.removeClass('empty-container');
-				else if ( $('#postbox-container-1').css('width') == '280px' )
+				else if ( $('.postbox-container-1').css('width') == '280px' )
 					side.addClass('empty-container');
 			}
 		},
Index: src/js/_enqueues/wp/editor/dfw.js
===================================================================
--- src/js/_enqueues/wp/editor/dfw.js	(revision 47239)
+++ src/js/_enqueues/wp/editor/dfw.js	(working copy)
@@ -31,7 +31,7 @@
 			$menuBar = $(),
 			$statusBar = $(),
 			$sideSortables = $( '#side-sortables' ),
-			$postboxContainer = $( '#postbox-container-1' ),
+			$postboxContainer = $( '.postbox-container-1' ),
 			$postBody = $('#post-body'),
 			fullscreen = window.wp.editor && window.wp.editor.fullscreen,
 			mceEditor,
Index: src/wp-admin/css/common.css
===================================================================
--- src/wp-admin/css/common.css	(revision 47239)
+++ src/wp-admin/css/common.css	(working copy)
@@ -2009,9 +2009,12 @@
 }
 
 .metabox-holder.columns-1 .postbox-container .empty-container,
-.columns-2 #postbox-container-3 .empty-container,
-.columns-2 #postbox-container-4 .empty-container,
-.columns-3 #postbox-container-4 .empty-container {
+.columns-2 .postbox-container-3 .empty-container,
+.columns-2 .postbox-container-4 .empty-container,
+.columns-3 .postbox-container-4 .empty-container,
+.columns-2 #postbox-container-3 .empty-container, /* Back-compat for pre 5.4 */
+.columns-2 #postbox-container-4 .empty-container, /* Back-compat for pre 5.4 */
+.columns-3 #postbox-container-4 .empty-container { /* Back-compat for pre 5.4 */
 	border: 0 none;
 	height: 0;
 	min-height: 0;
@@ -2023,7 +2026,8 @@
 	float: left;
 }
 
-#post-body.columns-2 #postbox-container-1 {
+#post-body.columns-2 .postbox-container-1,
+#post-body.columns-2 #postbox-container-1  { /* Back-compat for pre 5.4 */
 	float: right;
 	margin-right: -300px;
 	width: 280px;
Index: src/wp-admin/css/dashboard.css
===================================================================
--- src/wp-admin/css/dashboard.css	(revision 47239)
+++ src/wp-admin/css/dashboard.css	(working copy)
@@ -7,8 +7,11 @@
 }
 
 #wpbody-content #dashboard-widgets.columns-2 .postbox-container-2,
-#wpbody-content #dashboard-widgets.columns-2 #postbox-container-3,
-#wpbody-content #dashboard-widgets.columns-2 #postbox-container-4 {
+#wpbody-content #dashboard-widgets.columns-2 .postbox-container-3,
+#wpbody-content #dashboard-widgets.columns-2 .postbox-container-4,
+#wpbody-content #dashboard-widgets.columns-2 #postbox-container-2, /* Back-compat for pre 5.4 */
+#wpbody-content #dashboard-widgets.columns-2 #postbox-container-3, /* Back-compat for pre 5.4 */
+#wpbody-content #dashboard-widgets.columns-2 #postbox-container-4  { /* Back-compat for pre 5.4 */
 	float: right;
 	width: 50.5%;
 }
@@ -17,12 +20,15 @@
 	width: 33.5%;
 }
 
-#wpbody-content #dashboard-widgets.columns-3 #postbox-container-1 {
+#wpbody-content #dashboard-widgets.columns-3 .postbox-container-1,
+#wpbody-content #dashboard-widgets.columns-3 #postbox-container-1 { /* Back-compat for pre 5.4 */
 	width: 33%;
 }
 
-#wpbody-content #dashboard-widgets.columns-3 #postbox-container-3,
-#wpbody-content #dashboard-widgets.columns-3 #postbox-container-4 {
+#wpbody-content #dashboard-widgets.columns-3 .postbox-container-3,
+#wpbody-content #dashboard-widgets.columns-3 .postbox-container-4,
+#wpbody-content #dashboard-widgets.columns-3 #postbox-container-3, /* Back-compat for pre 5.4 */
+#wpbody-content #dashboard-widgets.columns-3 #postbox-container-4 { /* Back-compat for pre 5.4 */
 	float: right;
 }
 
@@ -34,7 +40,8 @@
 	width: 25%;
 }
 
-#dashboard-widgets-wrap .columns-3 #postbox-container-4 .empty-container {
+#dashboard-widgets-wrap .columns-3 .postbox-container-4 .empty-container,
+#dashboard-widgets-wrap .columns-3 #postbox-container-4 .empty-container { /* Back-compat for pre 5.4 */
 	border: none !important;
 }
 
@@ -43,14 +50,19 @@
 }
 
 .ie8 #wpbody-content #dashboard-widgets .postbox-container-2,
-.ie8 #wpbody-content #dashboard-widgets #postbox-container-3,
-.ie8 #wpbody-content #dashboard-widgets #postbox-container-4 {
+.ie8 #wpbody-content #dashboard-widgets .postbox-container-3,
+.ie8 #wpbody-content #dashboard-widgets .postbox-container-4,
+.ie8 #wpbody-content #dashboard-widgets #postbox-container-2, /* Back-compat for pre 5.4 */
+.ie8 #wpbody-content #dashboard-widgets #postbox-container-3, /* Back-compat for pre 5.4 */
+.ie8 #wpbody-content #dashboard-widgets #postbox-container-4  { /* Back-compat for pre 5.4 */
 	float: right;
 	width: 50.5%;
 }
 
-.ie8 #dashboard-widgets #postbox-container-3 .empty-container,
-.ie8 #dashboard-widgets #postbox-container-4 .empty-container {
+.ie8 #dashboard-widgets .postbox-container-3 .empty-container,
+.ie8 #dashboard-widgets .postbox-container-4 .empty-container,
+.ie8 #dashboard-widgets #postbox-container-3 .empty-container, /* Back-compat for pre 5.4 */
+.ie8 #dashboard-widgets #postbox-container-4 .empty-container  { /* Back-compat for pre 5.4 */
 	border: 0 none;
 	height: 0;
 	min-height: 0;
@@ -1113,21 +1125,28 @@
 	}
 
 	#wpbody-content #dashboard-widgets .postbox-container-2,
-	#wpbody-content #dashboard-widgets #postbox-container-3,
-	#wpbody-content #dashboard-widgets #postbox-container-4 {
+	#wpbody-content #dashboard-widgets .postbox-container-3,
+	#wpbody-content #dashboard-widgets .postbox-container-4,
+	#wpbody-content #dashboard-widgets #postbox-container-2, /* Back-compat for pre 5.4 */
+	#wpbody-content #dashboard-widgets #postbox-container-3, /* Back-compat for pre 5.4 */
+	#wpbody-content #dashboard-widgets #postbox-container-4  { /* Back-compat for pre 5.4 */
 		float: right;
 		width: 50.5%;
 	}
 
-	#dashboard-widgets #postbox-container-3 .empty-container,
-	#dashboard-widgets #postbox-container-4 .empty-container {
+	#dashboard-widgets .postbox-container-3 .empty-container,
+	#dashboard-widgets .postbox-container-4 .empty-container,
+	#dashboard-widgets #postbox-container-3 .empty-container, /* Back-compat for pre 5.4 */
+	#dashboard-widgets #postbox-container-4 .empty-container { /* Back-compat for pre 5.4 */
 		border: 0 none;
 		height: 0;
 		min-height: 0;
 	}
 
-	#dashboard-widgets #postbox-container-3 .empty-container:after,
-	#dashboard-widgets #postbox-container-4 .empty-container:after {
+	#dashboard-widgets .postbox-container-3 .empty-container:after,
+	#dashboard-widgets .postbox-container-4 .empty-container:after,
+	#dashboard-widgets #postbox-container-3 .empty-container:after, /* Back-compat for pre 5.4 */
+	#dashboard-widgets #postbox-container-4 .empty-container:after { /* Back-compat for pre 5.4 */
 		display: none;
 	}
 
@@ -1159,26 +1178,31 @@
 
 /* three columns on the dash */
 @media only screen and (min-width: 1500px) and (max-width: 1800px) {
-	#wpbody-content #dashboard-widgets .postbox-container {
+	#wpbody-content #dashboard-widgets .postbox-container  {
 		width: 33.5%;
 	}
 
-	#wpbody-content #dashboard-widgets #postbox-container-1 {
+	#wpbody-content #dashboard-widgets .postbox-container-1,
+	#wpbody-content #dashboard-widgets #postbox-container-1 { /* Back-compat for pre 5.4 */
 		width: 33%;
 	}
 
-	#wpbody-content #dashboard-widgets #postbox-container-3,
-	#wpbody-content #dashboard-widgets #postbox-container-4 {
+	#wpbody-content #dashboard-widgets .postbox-container-3,
+	#wpbody-content #dashboard-widgets .postbox-container-4,
+	#wpbody-content #dashboard-widgets #postbox-container-3, /* Back-compat for pre 5.4 */
+	#wpbody-content #dashboard-widgets #postbox-container-4 { /* Back-compat for pre 5.4 */
 		float: right;
 	}
 
-	#dashboard-widgets #postbox-container-4 .empty-container {
+	#dashboard-widgets .postbox-container-4 .empty-container,
+	#dashboard-widgets #postbox-container-4 .empty-container  { /* Back-compat for pre 5.4 */
 		border: 0 none;
 		height: 0;
 		min-height: 0;
 	}
 
-	#dashboard-widgets #postbox-container-4 .empty-container:after {
+	#dashboard-widgets .postbox-container-4 .empty-container:after,
+	#dashboard-widgets #postbox-container-4 .empty-container:after { /* Back-compat for pre 5.4 */
 		display: none;
 	}
 
Index: src/wp-admin/css/edit.css
===================================================================
--- src/wp-admin/css/edit.css	(revision 47239)
+++ src/wp-admin/css/edit.css	(working copy)
@@ -1,17 +1,21 @@
-.poststuff {
+.poststuff,
+#poststuff { /* Back-compat for pre 5.4 */
 	padding-top: 10px;
 	min-width: 763px;
 }
 
-.poststuff #post-body {
+.poststuff #post-body,
+#poststuff #post-body { /* Back-compat for pre 5.4 */
 	padding: 0;
 }
 
-.poststuff .postbox-container {
+.poststuff .postbox-container,
+#poststuff .postbox-container { /* Back-compat for pre 5.4 */
 	width: 100%;
 }
 
-.poststuff #post-body.columns-2 {
+.poststuff #post-body.columns-2,
+#poststuff #post-body.columns-2 { /* Back-compat for pre 5.4 */
 	margin-right: 300px;
 }
 
@@ -51,7 +55,8 @@
 	margin: 0;
 }
 
-.poststuff #titlewrap {
+.poststuff #titlewrap,
+#poststuff #titlewrap { /* Back-compat for pre 5.4 */
 	border: 0;
 	padding: 0;
 }
@@ -414,7 +419,8 @@
 	padding-top: 20px;
 }
 
-.poststuff #post-body.columns-2 #side-sortables {
+.poststuff #post-body.columns-2 #side-sortables,
+#poststuff #post-body.columns-2 #side-sortables { /* Back-compat for pre 5.4 */
 	width: 280px;
 }
 
@@ -598,32 +604,42 @@
 
 .poststuff h3.hndle, /* Back-compat for pre-4.4 */
 .poststuff .stuffbox > h3, /* Back-compat for pre-4.4 */
-.poststuff h2 {
+.poststuff h2,
+#poststuff h3.hndle, /* Back-compat for pre-5.4 */
+#poststuff .stuffbox > h3, /* Back-compat for pre-5.4 */
+#poststuff h2  { 
 	font-size: 14px;
 	padding: 8px 12px;
 	margin: 0;
 	line-height: 1.4;
 }
 
-.poststuff .stuffbox h2 {
+.poststuff .stuffbox h2,
+#poststuff .stuffbox h2  { /* Back-compat for pre-5.4 */
 	padding: 8px 10px;
 }
 
-.poststuff .inside {
+.poststuff .inside,
+#poststuff .inside { /* Back-compat for pre-5.4 */
 	margin: 6px 0 0 0;
 }
 
-.poststuff .stuffbox .inside {
+.poststuff .stuffbox .inside,
+#poststuff .stuffbox .inside { /* Back-compat for pre-5.4 */
 	margin: 0;
 }
 
 .poststuff .inside #parent_id,
-.poststuff .inside #page_template {
+.poststuff .inside #page_template,
+#poststuff .inside #parent_id, /* Back-compat for pre-5.4 */
+#poststuff .inside #page_template { /* Back-compat for pre-5.4 */
 	max-width: 100%;
 }
 
 .ie8 .poststuff .inside #parent_id,
-.ie8 .poststuff .inside #page_template {
+.ie8 .poststuff .inside #page_template,
+.ie8 #poststuff .inside #parent_id, /* Back-compat for pre-5.4 */
+.ie8 #poststuff .inside #page_template { /* Back-compat for pre-5.4 */
 	width: 250px;
 }
 
@@ -642,7 +658,8 @@
 }
 
 #linksubmitdiv .inside, /* Old Link Manager back-compat. */
-.poststuff #submitdiv .inside {
+.poststuff #submitdiv .inside,
+#poststuff #submitdiv .inside { /* Back-compat for pre-5.4 */
 	margin: 0;
 	padding: 0;
 }
@@ -1090,12 +1107,13 @@
 /*------------------------------------------------------------------------------
   13.0 - Tags
 ------------------------------------------------------------------------------*/
-
+#poststuff .tagsdiv .ajaxtag, /* Back-compat for pre 5.4 */
 .poststuff .tagsdiv .ajaxtag {
 	margin-top: 1em;
 }
 
-.poststuff .tagsdiv .howto {
+#poststuff .tagsdiv .howto, /* Back-compat for pre 5.4 */
+.poststuff .tagsdiv .howto  {
 	margin: 1em 0 6px 0;
 }
 
@@ -1140,6 +1158,7 @@
 	margin: 2px 0 12px;
 }
 
+#poststuff .inside .the-tagcloud, /* Back-compat for pre 5.4 */
 .poststuff .inside .the-tagcloud {
 	margin: 5px 0 10px;
 	padding: 8px;
@@ -1412,26 +1431,32 @@
 
 /* one column on the post write/edit screen */
 @media only screen and (max-width: 850px) {
+	#poststuff, /* Back-compat for pre 5.4 */
 	.poststuff {
 		min-width: 0;
 	}
 
+	#wpbody-content #poststuff #post-body, /* Back-compat for pre 5.4 */
 	#wpbody-content .poststuff #post-body {
 		margin: 0;
 	}
 
-	#wpbody-content #post-body.columns-2 #postbox-container-1 {
+	#wpbody-content #post-body.columns-2 #postbox-container-1, /* Back-compat for pre 5.4 */
+	#wpbody-content #post-body.columns-2 .postbox-container-1 {
 		margin-right: 0;
 		width: 100%;
 	}
 
-	.poststuff #postbox-container-1 .empty-container,
-	.poststuff #postbox-container-1 #side-sortables:empty {
+	.poststuff #postbox-container-1 .empty-container, /* Back-compat for pre 5.4 */
+	.poststuff #postbox-container-1 #side-sortables:empty, /* Back-compat for pre 5.4 */
+	.poststuff .postbox-container-1 .empty-container,
+	.poststuff .postbox-container-1 #side-sortables:empty {
 		border: 0 none;
 		height: 0;
 		min-height: 0;
 	}
 
+	#poststuff #post-body.columns-2 #side-sortables, /* Back-compat for pre 5.4 */
 	.poststuff #post-body.columns-2 #side-sortables {
 		min-height: 0;
 		width: auto;
@@ -1461,6 +1486,7 @@
 		padding: 0 2px 4px 0;
 	}
 
+	#poststuff h2, /* Back-compat for pre 5.4 */
 	.poststuff h3.hndle, /* Back-compat for pre-4.4 */
 	.poststuff .stuffbox > h3, /* Back-compat for pre-4.4 */
 	.poststuff h2 {
Index: src/wp-admin/css/ie.css
===================================================================
--- src/wp-admin/css/ie.css	(revision 47239)
+++ src/wp-admin/css/ie.css	(working copy)
@@ -13,15 +13,20 @@
 	width: 49.5%;
 }
 
+#wpbody-content #dashboard-widgets #postbox-container-2, /* Back-compat for pre 5.4 */
+#wpbody-content #dashboard-widgets #postbox-container-3, /* Back-compat for pre 5.4 */
+#wpbody-content #dashboard-widgets #postbox-container-4, /* Back-compat for pre 5.4 */
 #wpbody-content #dashboard-widgets .postbox-container-2,
-#wpbody-content #dashboard-widgets #postbox-container-3,
-#wpbody-content #dashboard-widgets #postbox-container-4 {
+#wpbody-content #dashboard-widgets .postbox-container-3,
+#wpbody-content #dashboard-widgets .postbox-container-4  {
 	float: right;
 	width: 50.5%;
 }
 
-#dashboard-widgets #postbox-container-3 .empty-container,
-#dashboard-widgets #postbox-container-4 .empty-container {
+#dashboard-widgets #postbox-container-3 .empty-container, /* Back-compat for pre 5.4 */
+#dashboard-widgets #postbox-container-4 .empty-container, /* Back-compat for pre 5.4 */
+#dashboard-widgets .postbox-container-3 .empty-container,
+#dashboard-widgets .postbox-container-4 .empty-container  {
 	border: 0 none;
 	height: 0;
 	min-height: 0;
@@ -38,7 +43,8 @@
 	width: 97%;
 }
 
-#post-body.columns-2 #postbox-container-1 {
+#post-body.columns-2 #postbox-container-1, /* Back-compat for pre 5.4 */
+#post-body.columns-2 .postbox-container-1 {
 	padding-left: 19px;
 }
 
Index: src/wp-admin/css/media.css
===================================================================
--- src/wp-admin/css/media.css	(revision 47239)
+++ src/wp-admin/css/media.css	(working copy)
@@ -1033,6 +1033,7 @@
 	display: block;
 }
 
+#poststuff .imgedit-group-top h2, /* Back-compat for pre 5.3 */
 .poststuff .imgedit-group-top h2 {
 	display: inline-block;
 	margin: 0;
@@ -1041,6 +1042,7 @@
 	line-height: 1.4;
 }
 
+#poststuff .imgedit-group-top .button-link, /* Back-compat for pre 5.3 */
 .poststuff .imgedit-group-top .button-link {
 	text-decoration: none;
 	color: #23282d;
Index: src/wp-admin/edit-form-advanced.php
===================================================================
--- src/wp-admin/edit-form-advanced.php	(revision 47239)
+++ src/wp-admin/edit-form-advanced.php	(working copy)
@@ -639,7 +639,7 @@
 ?>
 </div><!-- /post-body-content -->
 
-<div id="postbox-container-1" class="postbox-container">
+<div class="postbox-container-1 postbox-container">
 <?php
 
 if ( 'page' == $post_type ) {
Index: src/wp-admin/edit-form-comment.php
===================================================================
--- src/wp-admin/edit-form-comment.php	(revision 47239)
+++ src/wp-admin/edit-form-comment.php	(working copy)
@@ -85,7 +85,7 @@
 </div>
 </div><!-- /post-body-content -->
 
-<div id="postbox-container-1" class="postbox-container">
+<div class="postbox-container-1 postbox-container">
 <div id="submitdiv" class="stuffbox" >
 <h2><?php _e( 'Status' ); ?></h2>
 <div class="inside">
Index: src/wp-admin/edit-link-form.php
===================================================================
--- src/wp-admin/edit-link-form.php	(revision 47239)
+++ src/wp-admin/edit-link-form.php	(working copy)
@@ -135,7 +135,7 @@
 </div>
 </div><!-- /post-body-content -->
 
-<div id="postbox-container-1" class="postbox-container">
+<div class="postbox-container-1 postbox-container">
 <?php
 
 /** This action is documented in wp-admin/includes/meta-boxes.php */
Index: src/wp-admin/includes/dashboard.php
===================================================================
--- src/wp-admin/includes/dashboard.php	(revision 47239)
+++ src/wp-admin/includes/dashboard.php	(working copy)
@@ -242,16 +242,16 @@
 
 	?>
 <div id="dashboard-widgets" class="metabox-holder<?php echo $columns_css; ?>">
-	<div id="postbox-container-1" class="postbox-container">
+	<div class="postbox-container-1 postbox-container">
 	<?php do_meta_boxes( $screen->id, 'normal', '' ); ?>
 	</div>
 	<div class="postbox-container-2 postbox-container">
 	<?php do_meta_boxes( $screen->id, 'side', '' ); ?>
 	</div>
-	<div id="postbox-container-3" class="postbox-container">
+	<div class="postbox-container-3 postbox-container">
 	<?php do_meta_boxes( $screen->id, 'column3', '' ); ?>
 	</div>
-	<div id="postbox-container-4" class="postbox-container">
+	<div class="postbox-container-4 postbox-container">
 	<?php do_meta_boxes( $screen->id, 'column4', '' ); ?>
 	</div>
 </div>