diff --git a/src/js/_enqueues/admin/postbox.js b/src/js/_enqueues/admin/postbox.js
index ff144777f9..919ac92659 100644
--- a/src/js/_enqueues/admin/postbox.js
+++ b/src/js/_enqueues/admin/postbox.js
@@ -421,7 +421,7 @@
 					break;
 				case 0:
 				case 180:
-					if ( $('#poststuff').length ) {
+					if ( $( '.poststuff' ).length ) {
 						this._pb_edit(1);
 					} else {
 						if ( !check.length || !check.is(':checked') )
diff --git a/src/wp-admin/css/dashboard.css b/src/wp-admin/css/dashboard.css
index eac01008e8..02cbf11db7 100644
--- a/src/wp-admin/css/dashboard.css
+++ b/src/wp-admin/css/dashboard.css
@@ -6,7 +6,7 @@
 	width: 49.5%;
 }
 
-#wpbody-content #dashboard-widgets.columns-2 #postbox-container-2,
+#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 {
 	float: right;
@@ -42,7 +42,7 @@
 	width: 49.5%;
 }
 
-.ie8 #wpbody-content #dashboard-widgets #postbox-container-2,
+.ie8 #wpbody-content #dashboard-widgets .postbox-container-2,
 .ie8 #wpbody-content #dashboard-widgets #postbox-container-3,
 .ie8 #wpbody-content #dashboard-widgets #postbox-container-4 {
 	float: right;
@@ -1112,7 +1112,7 @@ a.rsswidget {
 		width: 49.5%;
 	}
 
-	#wpbody-content #dashboard-widgets #postbox-container-2,
+	#wpbody-content #dashboard-widgets .postbox-container-2,
 	#wpbody-content #dashboard-widgets #postbox-container-3,
 	#wpbody-content #dashboard-widgets #postbox-container-4 {
 		float: right;
diff --git a/src/wp-admin/css/edit.css b/src/wp-admin/css/edit.css
index 656b953058..9ce2f72a5f 100644
--- a/src/wp-admin/css/edit.css
+++ b/src/wp-admin/css/edit.css
@@ -1,17 +1,17 @@
-#poststuff {
+.poststuff {
 	padding-top: 10px;
 	min-width: 763px;
 }
 
-#poststuff #post-body {
+.poststuff #post-body {
 	padding: 0;
 }
 
-#poststuff .postbox-container {
+.poststuff .postbox-container {
 	width: 100%;
 }
 
-#poststuff #post-body.columns-2 {
+.poststuff #post-body.columns-2 {
 	margin-right: 300px;
 }
 
@@ -51,7 +51,7 @@
 	margin: 0;
 }
 
-#poststuff #titlewrap {
+.poststuff #titlewrap {
 	border: 0;
 	padding: 0;
 }
@@ -414,7 +414,7 @@ form#tags-filter {
 	padding-top: 20px;
 }
 
-#poststuff #post-body.columns-2 #side-sortables {
+.poststuff #post-body.columns-2 #side-sortables {
 	width: 280px;
 }
 
@@ -596,34 +596,34 @@ form#tags-filter {
 	text-indent: 0;
 }
 
-#poststuff h3.hndle, /* Back-compat for pre-4.4 */
-#poststuff .stuffbox > h3, /* Back-compat for pre-4.4 */
-#poststuff h2 {
+.poststuff h3.hndle, /* Back-compat for pre-4.4 */
+.poststuff .stuffbox > h3, /* Back-compat for pre-4.4 */
+.poststuff h2 {
 	font-size: 14px;
 	padding: 8px 12px;
 	margin: 0;
 	line-height: 1.4;
 }
 
-#poststuff .stuffbox h2 {
+.poststuff .stuffbox h2 {
 	padding: 8px 10px;
 }
 
-#poststuff .inside {
+.poststuff .inside {
 	margin: 6px 0 0 0;
 }
 
-#poststuff .stuffbox .inside {
+.poststuff .stuffbox .inside {
 	margin: 0;
 }
 
-#poststuff .inside #parent_id,
-#poststuff .inside #page_template {
+.poststuff .inside #parent_id,
+.poststuff .inside #page_template {
 	max-width: 100%;
 }
 
-.ie8 #poststuff .inside #parent_id,
-.ie8 #poststuff .inside #page_template {
+.ie8 .poststuff .inside #parent_id,
+.ie8 .poststuff .inside #page_template {
 	width: 250px;
 }
 
@@ -642,7 +642,7 @@ form#tags-filter {
 }
 
 #linksubmitdiv .inside, /* Old Link Manager back-compat. */
-#poststuff #submitdiv .inside {
+.poststuff #submitdiv .inside {
 	margin: 0;
 	padding: 0;
 }
@@ -1091,11 +1091,11 @@ p.description code {
   13.0 - Tags
 ------------------------------------------------------------------------------*/
 
-#poststuff .tagsdiv .ajaxtag {
+.poststuff .tagsdiv .ajaxtag {
 	margin-top: 1em;
 }
 
-#poststuff .tagsdiv .howto {
+.poststuff .tagsdiv .howto {
 	margin: 1em 0 6px 0;
 }
 
@@ -1140,7 +1140,7 @@ p.popular-tags a {
 	margin: 2px 0 12px;
 }
 
-#poststuff .inside .the-tagcloud {
+.poststuff .inside .the-tagcloud {
 	margin: 5px 0 10px;
 	padding: 8px;
 	border: 1px solid #ddd;
@@ -1412,11 +1412,11 @@ table.links-table {
 
 /* one column on the post write/edit screen */
 @media only screen and (max-width: 850px) {
-	#poststuff {
+	.poststuff {
 		min-width: 0;
 	}
 
-	#wpbody-content #poststuff #post-body {
+	#wpbody-content .poststuff #post-body {
 		margin: 0;
 	}
 
@@ -1425,14 +1425,14 @@ table.links-table {
 		width: 100%;
 	}
 
-	#poststuff #postbox-container-1 .empty-container,
-	#poststuff #postbox-container-1 #side-sortables:empty {
+	.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 {
+	.poststuff #post-body.columns-2 #side-sortables {
 		min-height: 0;
 		width: auto;
 	}
@@ -1457,13 +1457,13 @@ table.links-table {
 		padding: 10px 10px;
 	}
 
-	#poststuff .stuffbox .inside {
+	.poststuff .stuffbox .inside {
 		padding: 0 2px 4px 0;
 	}
 
-	#poststuff h3.hndle, /* Back-compat for pre-4.4 */
-	#poststuff .stuffbox > h3, /* Back-compat for pre-4.4 */
-	#poststuff h2 {
+	.poststuff h3.hndle, /* Back-compat for pre-4.4 */
+	.poststuff .stuffbox > h3, /* Back-compat for pre-4.4 */
+	.poststuff h2 {
 		padding: 12px;
 	}
 
diff --git a/src/wp-admin/css/ie.css b/src/wp-admin/css/ie.css
index 00853ab7c8..89a6d38629 100644
--- a/src/wp-admin/css/ie.css
+++ b/src/wp-admin/css/ie.css
@@ -13,7 +13,7 @@
 	width: 49.5%;
 }
 
-#wpbody-content #dashboard-widgets #postbox-container-2,
+#wpbody-content #dashboard-widgets .postbox-container-2,
 #wpbody-content #dashboard-widgets #postbox-container-3,
 #wpbody-content #dashboard-widgets #postbox-container-4 {
 	float: right;
@@ -367,7 +367,7 @@ a.button.button-small {
 #wrap,
 #postdivrich,
 #postdiv,
-#poststuff,
+.poststuff,
 .metabox-holder,
 #titlediv,
 #post-body,
@@ -414,12 +414,12 @@ form#template div {
 	padding: 0 4px;
 }
 
-#poststuff h2 {
+.poststuff h2 {
 	font-size: 1.6em;
 }
 
-#poststuff .inside #parent_id,
-#poststuff .inside #page_template,
+.poststuff .inside #parent_id,
+.poststuff .inside #page_template,
 .inline-edit-row #post_parent,
 .inline-edit-row select[name="page_template"] {
 	width: 250px;
@@ -677,7 +677,7 @@ input[type="password"],
 	padding: 0;
 }
 
-* html #poststuff h2 {
+* html .poststuff h2 {
 	margin-left: 0;
 }
 
diff --git a/src/wp-admin/css/media.css b/src/wp-admin/css/media.css
index b945fa34cd..6906ef6467 100644
--- a/src/wp-admin/css/media.css
+++ b/src/wp-admin/css/media.css
@@ -1033,7 +1033,7 @@ border color while dragging a file over the uploader drop area */
 	display: block;
 }
 
-#poststuff .imgedit-group-top h2 {
+.poststuff .imgedit-group-top h2 {
 	display: inline-block;
 	margin: 0;
 	padding: 0;
@@ -1041,7 +1041,7 @@ border color while dragging a file over the uploader drop area */
 	line-height: 1.4;
 }
 
-#poststuff .imgedit-group-top .button-link {
+.poststuff .imgedit-group-top .button-link {
 	text-decoration: none;
 	color: #23282d;
 }
diff --git a/src/wp-admin/edit-form-advanced.php b/src/wp-admin/edit-form-advanced.php
index 60d920729d..13d4046c03 100644
--- a/src/wp-admin/edit-form-advanced.php
+++ b/src/wp-admin/edit-form-advanced.php
@@ -494,7 +494,7 @@ wp_nonce_field( 'closedpostboxes', 'closedpostboxesnonce', false );
 do_action( 'edit_form_top', $post );
 ?>
 
-<div id="poststuff">
+<div class="poststuff">
 <div id="post-body" class="metabox-holder columns-<?php echo 1 == get_current_screen()->get_columns() ? '1' : '2'; ?>">
 <div id="post-body-content">
 
@@ -671,7 +671,7 @@ do_meta_boxes( $post_type, 'side', $post );
 
 ?>
 </div>
-<div id="postbox-container-2" class="postbox-container">
+<div class="postbox-container-2 postbox-container">
 <?php
 
 do_meta_boxes( null, 'normal', $post );
diff --git a/src/wp-admin/edit-form-comment.php b/src/wp-admin/edit-form-comment.php
index 43b568a0aa..c6b379b7d1 100644
--- a/src/wp-admin/edit-form-comment.php
+++ b/src/wp-admin/edit-form-comment.php
@@ -16,7 +16,7 @@ if ( ! defined( 'ABSPATH' ) ) {
 <div class="wrap">
 <h1><?php _e( 'Edit Comment' ); ?></h1>
 
-<div id="poststuff">
+<div class="poststuff">
 <input type="hidden" name="action" value="editedcomment" />
 <input type="hidden" name="comment_ID" value="<?php echo esc_attr( $comment->comment_ID ); ?>" />
 <input type="hidden" name="comment_post_ID" value="<?php echo esc_attr( $comment->comment_post_ID ); ?>" />
@@ -196,7 +196,7 @@ endif;
 </div><!-- /submitdiv -->
 </div>
 
-<div id="postbox-container-2" class="postbox-container">
+<div class="postbox-container-2 postbox-container">
 <?php
 /** This action is documented in wp-admin/includes/meta-boxes.php */
 do_action( 'add_meta_boxes', 'comment', $comment );
diff --git a/src/wp-admin/edit-link-form.php b/src/wp-admin/edit-link-form.php
index f71684de63..d2733282c0 100644
--- a/src/wp-admin/edit-link-form.php
+++ b/src/wp-admin/edit-link-form.php
@@ -106,7 +106,7 @@ wp_nonce_field( 'closedpostboxes', 'closedpostboxesnonce', false );
 wp_nonce_field( 'meta-box-order', 'meta-box-order-nonce', false );
 ?>
 
-<div id="poststuff">
+<div class="poststuff">
 
 <div id="post-body" class="metabox-holder columns-<?php echo 1 == get_current_screen()->get_columns() ? '1' : '2'; ?>">
 <div id="post-body-content">
@@ -144,7 +144,7 @@ $side_meta_boxes = do_meta_boxes( 'link', 'side', $link );
 
 ?>
 </div>
-<div id="postbox-container-2" class="postbox-container">
+<div class="postbox-container-2 postbox-container">
 <?php
 
 do_meta_boxes( null, 'normal', $link );
diff --git a/src/wp-admin/includes/dashboard.php b/src/wp-admin/includes/dashboard.php
index e64897a8a6..7fbcd893cf 100644
--- a/src/wp-admin/includes/dashboard.php
+++ b/src/wp-admin/includes/dashboard.php
@@ -245,7 +245,7 @@ function wp_dashboard() {
 	<div id="postbox-container-1" class="postbox-container">
 	<?php do_meta_boxes( $screen->id, 'normal', '' ); ?>
 	</div>
-	<div id="postbox-container-2" class="postbox-container">
+	<div class="postbox-container-2 postbox-container">
 	<?php do_meta_boxes( $screen->id, 'side', '' ); ?>
 	</div>
 	<div id="postbox-container-3" class="postbox-container">
diff --git a/src/wp-admin/includes/post.php b/src/wp-admin/includes/post.php
index 1e8b20e668..d9ef63f7b2 100644
--- a/src/wp-admin/includes/post.php
+++ b/src/wp-admin/includes/post.php
@@ -2271,8 +2271,8 @@ function the_block_editor_meta_boxes() {
 	</form>
 	<?php foreach ( $locations as $location ) : ?>
 		<form class="metabox-location-<?php echo esc_attr( $location ); ?>" onsubmit="return false;">
-			<div id="poststuff" class="sidebar-open">
-				<div id="postbox-container-2" class="postbox-container">
+			<div class="poststuff" class="sidebar-open">
+				<div class="postbox-container-2 postbox-container">
 					<?php
 					do_meta_boxes(
 						$current_screen,