Escape \ in unicode escape sequence strings

Author: sirreal

src/wp-includes/html-api/class-wp-html-tag-processor.php

@@ -3765,8 +3765,8 @@ public function set_modifiable_text( string $plaintext_content ): bool {
 							'~<(/?)(s)(cript)([\t\r\n\f />])~i',
 							static function ( $matches ) {
 								$escaped_s_char = 's' === $matches[2]
-									? '\u0073'
-									: '\u0053';
+									? '\\u0073'
+									: '\\u0053';
 								return "<{$matches[1]}{$escaped_s_char}{$matches[3]}{$matches[4]}";
 							},
 							$plaintext_content