Hello!
From PSR-12:
https://www.php-fig.org/psr/psr-12/#3-declare-statements-namespace-and-import-statements
When wishing to declare strict types in files containing markup outside PHP opening and closing tags, the declaration MUST be on the first line of the file and include an opening PHP tag, the strict types declaration and closing tag.
For example:
<?php declare(strict_types=1) ?>
<html>
<body>
<?php
// ... additional PHP code ...
?>
</body>
</html>
But PHP_CodeSniffer show the error on the first line of this example code:
phpcs: PSR12.Files.FileHeader.SpacingAfterBlock: Header blocks must be separated by a single blank line