diff --git a/wp-admin/options-reading.php b/wp-admin/options-reading.php
index 5dd40f37c4..8f3ba83a5f 100644
--- a/wp-admin/options-reading.php
+++ b/wp-admin/options-reading.php
@@ -64,7 +64,7 @@ require_once ABSPATH . 'wp-admin/admin-header.php';
 <?php
 settings_fields( 'reading' );
 
-if ( ! in_array( get_option( 'blog_charset' ), array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ), true ) ) {
+if ( ! is_utf8_charset() ) {
 	add_settings_field( 'blog_charset', __( 'Encoding for pages and feeds' ), 'options_reading_blog_charset', 'reading', 'default', array( 'label_for' => 'blog_charset' ) );
 }
 ?>
diff --git a/wp-admin/options.php b/wp-admin/options.php
index 8510095e24..34b98ae294 100644
--- a/wp-admin/options.php
+++ b/wp-admin/options.php
@@ -160,7 +160,7 @@ $allowed_options['privacy'] = array();
 
 $mail_options = array( 'mailserver_url', 'mailserver_port', 'mailserver_login', 'mailserver_pass' );
 
-if ( ! in_array( get_option( 'blog_charset' ), array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ), true ) ) {
+if ( ! is_utf8_charset() ) {
 	$allowed_options['reading'][] = 'blog_charset';
 }
 
diff --git a/wp-includes/compat.php b/wp-includes/compat.php
index b4ef3d2f67..c50fc69a04 100644
--- a/wp-includes/compat.php
+++ b/wp-includes/compat.php
@@ -91,7 +91,7 @@ function _mb_substr( $str, $start, $length = null, $encoding = null ) {
 	 * The solution below works only for UTF-8, so in case of a different
 	 * charset just use built-in substr().
 	 */
-	if ( ! in_array( $encoding, array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ), true ) ) {
+	if ( ! is_utf8_charset( $encoding ) ) {
 		return is_null( $length ) ? substr( $str, $start ) : substr( $str, $start, $length );
 	}
 
@@ -176,7 +176,7 @@ function _mb_strlen( $str, $encoding = null ) {
 	 * The solution below works only for UTF-8, so in case of a different charset
 	 * just use built-in strlen().
 	 */
-	if ( ! in_array( $encoding, array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ), true ) ) {
+	if ( ! is_utf8_charset( $encoding ) ) {
 		return strlen( $str );
 	}
 
diff --git a/wp-includes/formatting.php b/wp-includes/formatting.php
index 77c772001c..7c03c484dd 100644
--- a/wp-includes/formatting.php
+++ b/wp-includes/formatting.php
@@ -960,19 +960,7 @@ function _wp_specialchars( $text, $quote_style = ENT_NOQUOTES, $charset = false,
 		$quote_style = ENT_QUOTES;
 	}
 
-	// Store the site charset as a static to avoid multiple calls to wp_load_alloptions().
-	if ( ! $charset ) {
-		static $_charset = null;
-		if ( ! isset( $_charset ) ) {
-			$alloptions = wp_load_alloptions();
-			$_charset   = isset( $alloptions['blog_charset'] ) ? $alloptions['blog_charset'] : '';
-		}
-		$charset = $_charset;
-	}
-
-	if ( in_array( $charset, array( 'utf8', 'utf-8', 'UTF8' ), true ) ) {
-		$charset = 'UTF-8';
-	}
+	$charset = _canonical_charset( $charset ? $charset : get_option( 'blog_charset' ) );
 
 	$_quote_style = $quote_style;
 
@@ -1114,7 +1102,7 @@ function wp_check_invalid_utf8( $text, $strip = false ) {
 	// Store the site charset as a static to avoid multiple calls to get_option().
 	static $is_utf8 = null;
 	if ( ! isset( $is_utf8 ) ) {
-		$is_utf8 = in_array( get_option( 'blog_charset' ), array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ), true );
+		$is_utf8 = is_utf8_charset();
 	}
 	if ( ! $is_utf8 ) {
 		return $text;
diff --git a/wp-includes/functions.php b/wp-includes/functions.php
index 0cc4c44ad1..3634190663 100644
--- a/wp-includes/functions.php
+++ b/wp-includes/functions.php
@@ -7474,17 +7474,27 @@ function get_tag_regex( $tag ) {
  *
  * @see https://core.trac.wordpress.org/ticket/23688
  *
- * @param string $charset A charset name.
+ * @param string $charset A charset name, e.g. "UTF-8", "Windows-1252", "SJIS".
  * @return string The canonical form of the charset.
  */
 function _canonical_charset( $charset ) {
-	if ( 'utf-8' === strtolower( $charset ) || 'utf8' === strtolower( $charset ) ) {
-
+	if ( is_utf8_charset( $charset ) ) {
 		return 'UTF-8';
 	}
 
-	if ( 'iso-8859-1' === strtolower( $charset ) || 'iso8859-1' === strtolower( $charset ) ) {
-
+	/*
+	 * Normalize the ISO-8859-1 family of languages.
+	 *
+	 * This is not required for htmlspecialchars(), as it properly recognizes all of
+	 * the input character sets that here are transformed into "ISO-8859-1".
+	 *
+	 * @todo Should this entire check be removed since it's not required for the stated purpose?
+	 * @todo Should WordPress transform other potential charset equivalents, such as "latin1"?
+	 */
+	if (
+		( 0 === strcasecmp( 'iso-8859-1', $charset ) ) ||
+		( 0 === strcasecmp( 'iso8859-1', $charset ) )
+	) {
 		return 'ISO-8859-1';
 	}
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 5263db6a26..418dcc4f9a 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6-alpha-58146';
+$wp_version = '6.6-alpha-58147';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-settings.php b/wp-settings.php
index 9673479bfa..4d8a35ae83 100644
--- a/wp-settings.php
+++ b/wp-settings.php
@@ -106,6 +106,7 @@ if ( WP_CACHE && apply_filters( 'enable_loading_advanced_cache_dropin', true ) &
 wp_set_lang_dir();
 
 // Load early WordPress files.
+require ABSPATH . WPINC . '/unicode.php';
 require ABSPATH . WPINC . '/class-wp-list-util.php';
 require ABSPATH . WPINC . '/formatting.php';
 require ABSPATH . WPINC . '/meta.php';