Sync string escapes with GB PR

Author: sirreal

src/wp-includes/css-api/class-wp-css-token-processor.php

@@ -856,6 +856,7 @@ private function create_css_string( string $value ): string {
 		$escaped = strtr(
 			$value,
 			array(
+				// Escape existing backslashes to prevent unintentional escapes in result.
 				'\\'   => '\\5C ',
 
 				// Pre-processing replaces NULLs and some newlines. Replace and escape as necessary.
@@ -869,6 +870,8 @@ private function create_css_string( string $value ): string {
 				// Newlines must be escaped in CSS strings.
 				"\n"   => '\\A ',
 
+				// Arbitrary characters for Unicode escaping:
+
 				// HTML syntax may be problematic.
 				'<'    => '\\3C ',
 				'>'    => '\\3E ',