Source: https://codex.wordpress.org/Commenting_Code

<!-- This is a comment in HTML, 
wrapped around the entire block -->Code language: HTML, XML (xml)

/* This is a comment in CSS files. Also used in PHP files outside the actual PHP code itself,
wrapped around the entire block */Code language: JSON / JSON with Comments (json)

# This is a comment in .htaccess
# Needed in front of each line Code language: PHP (php)

<?php

// This is a comment inside PHP code 
// Needed in front of each line
// Can also be added as the end of line with PHP code

?>Code language: HTML, XML (xml)