Index: src/wp-admin/includes/additional-css.php
===================================================================
--- src/wp-admin/includes/additional-css.php	(nonexistent)
+++ src/wp-admin/includes/additional-css.php	(working copy)
@@ -0,0 +1,59 @@
+<?php
+include_once ABSPATH . 'wp-admin/includes/class-additional-css-to-array.php';
+
+/**
+ * Additional CSS to the block editor
+ */
+add_action( 'enqueue_block_editor_assets', 'enqueue_block_editor_assets_admin_head' );
+
+function enqueue_block_editor_assets_admin_head() {
+	add_action( 'admin_head', 'additional_css_to_block_editor' );
+}
+
+function additional_css_to_block_editor() {
+	$css = wp_get_custom_css();
+	$css_to_array = new Additional_CSS_To_Array( $css );
+	$css = $css_to_array->get();
+
+	foreach ( $css as $key => $css_block ) {
+		$selectors = $css_block->get_selectors();
+		foreach ( $selectors as $i => $selector ) {
+			$selectors[ $i ] = '.editor-styles-wrapper ' . $selector;
+		}
+		$css_block->set_selectors( $selectors );
+		$css[ $key ] = $css_block;
+	}
+
+	$new_css = '';
+	foreach ( $css as $key => $css_block ) {
+		$new_css .= $css_block->get_inline_css();
+	}
+	?>
+	<style id="wp-additional-css">
+	<?php echo strip_tags( $new_css ); // WPCS XSS ok. ?>
+	</style>
+	<?php
+}
+
+/**
+ * Additional CSS to the classic editor
+ */
+add_filter( 'tiny_mce_before_init', 'additional_css_to_classic_editor' );
+
+function additional_css_to_classic_editor( $mce_init ) {
+	$css = wp_get_custom_css();
+	$css_to_array = new Additional_CSS_To_Array( $css );
+	$css = $css_to_array->get();
+
+	$new_css = '';
+	foreach ( $css as $key => $css_block ) {
+		$new_css .= $css_block->get_inline_css();
+	}
+
+	if ( ! isset( $mce_init['content_style'] ) ) {
+		$mce_init['content_style'] = '';
+	}
+
+	$mce_init['content_style'] .= addslashes( $new_css );
+	return $mce_init;
+}
Index: src/wp-admin/includes/admin.php
===================================================================
--- src/wp-admin/includes/admin.php	(revision 45254)
+++ src/wp-admin/includes/admin.php	(working copy)
@@ -16,6 +16,9 @@
 	load_textdomain( 'default', WP_LANG_DIR . '/admin-' . get_locale() . '.mo' );
 }
 
+/** Additional CSS API */
+require_once( ABSPATH . 'wp-admin/includes/additional-css.php' );
+
 /** WordPress Administration Hooks */
 require_once( ABSPATH . 'wp-admin/includes/admin-filters.php' );
 
Index: src/wp-admin/includes/class-additional-css-to-array.php
===================================================================
--- src/wp-admin/includes/class-additional-css-to-array.php	(nonexistent)
+++ src/wp-admin/includes/class-additional-css-to-array.php	(working copy)
@@ -0,0 +1,97 @@
+<?php
+/**
+ * Additinal CSS API
+ *
+ * @package WordPress
+ * @subpackage Administration
+ */
+
+/**
+ * The additional CSS reflect to the editor
+ */
+class Additional_CSS_To_Array {
+
+	/**
+	 * CSS
+	 *
+	 * @var string
+	 */
+	protected $css;
+
+	/**
+	 * @param string $css
+	 */
+	public function __construct( $css ) {
+	include_once ABSPATH . 'wp-admin/includes/class-additional-css.php';
+		$this->css = $this->_convert( $css );
+	}
+
+	/**
+	 * @return string
+	 */
+	public function get() {
+		return $this->css;
+	}
+
+	/**
+	 * Convert inline css to PHP array
+	 *
+	 * @param string $css
+	 * @return array
+	 */
+	protected function _convert( $css ) {
+		$css = $this->_clean( $css );
+		$css = explode( '}', $css );
+		$css = array_filter( $css, 'strlen' );
+
+		foreach ( $css as $key => $val ) {
+			$css[ $key ] = explode( '{', $val );
+		}
+
+		foreach ( $css as $key => $css_block ) {
+			$css[ $key ] = $this->_create_css_block( $css_block );
+		}
+
+		return $css;
+	}
+
+	/**
+	 * Create CSS_Block
+	 *
+	 * @param array $css_block
+	 * @return Additional_CSS
+	 */
+	protected function _create_css_block( $css_block ) {
+		if ( $this->_has_pre_selector( $css_block ) ) {
+			$css_block     = array_reverse( $css_block );
+			$selectors     = explode( ',', $css_block[1] );
+			$properties    = $css_block[0];
+			$pre_selectors = array_slice( $css_block, 2 );
+		} else {
+			$selectors     = explode( ',', $css_block[0] );
+			$properties    = $css_block[1];
+			$pre_selectors = [];
+		}
+		return new Additional_CSS( $selectors, $properties, $pre_selectors );
+	}
+
+	/**
+	 * Return true when has pre selector
+	 *
+	 * @param array $css_block
+	 * @return boolean
+	 */
+	protected function _has_pre_selector( $css_block ) {
+		return 2 < count( $css_block );
+	}
+
+	/**
+	 * Remove tab and line breaks
+	 *
+	 * @param string $value
+	 * @return string
+	 */
+	protected function _clean( $value ) {
+		return str_replace( [ "\t", "\n", "\r" ], '', $value );
+	}
+}
Index: src/wp-admin/includes/class-additional-css.php
===================================================================
--- src/wp-admin/includes/class-additional-css.php	(nonexistent)
+++ src/wp-admin/includes/class-additional-css.php	(working copy)
@@ -0,0 +1,110 @@
+<?php
+/**
+ * Additinal CSS API
+ *
+ * @package WordPress
+ * @subpackage Administration
+ */
+
+/**
+ * The additional CSS reflect to the editor
+ */
+class Additional_CSS {
+
+	/**
+	 * Selectors
+	 *
+	 * @var array
+	 */
+	protected $selectors = [];
+
+	/**
+	 * CSS properties
+	 *
+	 * @var string
+	 */
+	protected $properties;
+
+	/**
+	 * Pre selectors
+	 *
+	 * @var array
+	 */
+	protected $pre_selectors = [];
+
+	/**
+	 * @param array $selectors
+	 * @param string $properties
+	 * @param array $pre_selectors
+	 */
+	public function __construct( array $selectors, $properties, array $pre_selectors = [] ) {
+		$this->selectors     = $selectors;
+		$this->properties    = $properties;
+		$this->pre_selectors = $pre_selectors;
+	}
+
+	/**
+	 * Return inline CSS
+	 *
+	 * @return string
+	 */
+	public function get_inline_css() {
+		if ( $this->get_pre_selectors() ) {
+			$inline_css = sprintf(
+				'%1$s { %2$s }',
+				implode( ',', $this->get_selectors() ),
+				$this->get_properties()
+			);
+			foreach ( $this->get_pre_selectors() as $pre_selector ) {
+				$inline_css = sprintf(
+					'%1$s { %2$s }',
+					$pre_selector,
+					$inline_css
+				);
+			}
+			return $inline_css;
+		}
+
+		return sprintf(
+			'%1$s { %2$s }',
+			implode( ',', $this->get_selectors() ),
+			$this->get_properties()
+		);
+	}
+
+	/**
+	 * Return selectors
+	 *
+	 * @return array
+	 */
+	public function get_selectors() {
+		return $this->selectors;
+	}
+
+	/**
+	 * Return CSS properties
+	 *
+	 * @return string
+	 */
+	public function get_properties() {
+		return $this->properties;
+	}
+
+	/**
+	 * Return pre selectors
+	 *
+	 * @return array
+	 */
+	public function get_pre_selectors() {
+		return $this->pre_selectors;
+	}
+
+	/**
+	 * Set selectors
+	 *
+	 * @param array
+	 */
+	public function set_selectors( array $selectors ) {
+		$this->selectors = $selectors;
+	}
+}