Inline critical CSS

<?php
/**
 * Inline Critical CSS
 *
 * Embeds a small CSS file directly in the <head> to style above-the-fold
 * content without waiting for an external stylesheet to download. This
 * eliminates the render-blocking delay on first paint. Keep the critical
 * CSS under 14 KB (one TCP round-trip) for maximum benefit.
 *
 * Usage: Add to functions.php. Create the critical CSS file at the path
 *        specified below. Generate it with tools like Critical or PurgeCSS.
 *
 * @category performance
 * @language php

 *
 * @author Adame Dahmani https://adame.io
 * @support Support my family project: https://adame.io/fundraiser
 */
add_action( 'wp_head', function () {
    $critical_path = get_template_directory() . '/assets/critical.css';

    if ( ! file_exists( $critical_path ) ) {
        return;
    }

    echo '<style id="critical-css">';
    echo file_get_contents( $critical_path );
    echo '</style>';
}, 1 );