diff --git a/wp-includes/template.php b/wp-includes/template.php
index f799113e9c..f17307e19d 100644
--- a/wp-includes/template.php
+++ b/wp-includes/template.php
@@ -926,13 +926,24 @@ function wp_finalize_template_enhancement_output_buffer( string $output, int $ph
 	$is_html_content_type = null;
 	$html_content_types   = array( 'text/html', 'application/xhtml+xml' );
 	foreach ( headers_list() as $header ) {
-		$header_parts = preg_split( '/\s*[:;]\s*/', strtolower( $header ) );
+		$header_parts = explode( ':', strtolower( $header ), 2 );
 		if (
-			is_array( $header_parts ) &&
-			count( $header_parts ) >= 2 &&
+			count( $header_parts ) === 2 &&
 			'content-type' === $header_parts[0]
 		) {
-			$is_html_content_type = in_array( $header_parts[1], $html_content_types, true );
+			/*
+			 * This is looking for very specific content types, therefore it
+			 * doesn’t need to fully parse the header’s value. Instead, it needs
+			 * only assert that the content type is one of the static HTML types.
+			 *
+			 * Example:
+			 *
+			 *     Content-Type: text/html; charset=utf8
+			 *     Content-Type: text/html  ;charset=latin4
+			 *     Content-Type:application/xhtml+xml
+			 */
+			$media_type           = trim( strtok( $header_parts[1], ';' ), " \t" );
+			$is_html_content_type = in_array( $media_type, $html_content_types, true );
 			break; // PHP only sends the first Content-Type header in the list.
 		}
 	}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 38e78755e5..1209fd5797 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.9-alpha-60972';
+$wp_version = '6.9-alpha-60973';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.