HTML comments may not work for debugging - Debug comments may not appear in markup if the page is minified. https://www.damiencarbery.com/2019/08/html-comments-may-not-work-for-debugging/

debug-with-comments.php

<?php
/*
Plugin Name: Debug with comments
Plugin URI: https://www.damiencarbery.com/2019/08/html-comments-may-not-work-for-debugging/
Description: Debug using html comments.
Author: Damien Carbery
Version: 0.1
*/

// This div *will* be removed during html minification.
add_action( 'wp_footer', 'dcwd_footer_debug_with_comments' );
function dcwd_footer_debug_with_comments() {
	echo '<!-- Page ID: ', get_the_id(), ' -->';
}

debug-with-hidden-div.php

<?php
/*
Plugin Name: Debug with hidden div
Plugin URI: https://www.damiencarbery.com/2019/08/html-comments-may-not-work-for-debugging/
Description: Debug using hidden div instead of html comments.
Author: Damien Carbery
Version: 0.1
*/

// This div will *not* be removed during html minification.
add_action( 'wp_footer', 'dcwd_footer_debug_with_hidden_div' );
function dcwd_footer_debug_with_hidden_div() {
	echo '<div style="display:none">Page ID: ', get_the_id(), ' </div>';
}

markup-after-minification.html

<p class="first-p">This is the first paragraph.</p><p class="last-p">This is the last paragraph.</p>

markup-before-minification.html

<!-- Comment before the content -->
<p class="first-p">This is the first paragraph.</p>
<!-- A comment in the middle -->
<p class="last-p">This is the last paragraph.</p>