WordPress Spam Pixel code for WP Forms

spam-pixel.php

<?php
/**
 * Plugin Name:     Spam Pixel
 * Description:     This simple plugin integrates Ross's spam pixel idea with WP Forms
 * Author:          Ross Wintle
 * Author URI:      https://rosswintle.uk
 * Text Domain:     spam-pixel
 * Domain Path:     /languages
 * Version:         0.1.0
 *
 * @package         Spam_Pixel
 */

// Add field and CSS
add_action('wpforms_display_submit_before', 'spx_add_field');

function spx_add_field($form_data) {
	$ajax_url = admin_url('admin-ajax.php') . '?action=spx_pixel';
	echo '<div class="spx-captcha"></div>';
	echo <<<EOT
		<style>
			.spx-captcha {
				width: 1px;
				height: 1px;
			}
			form:focus-within .spx-captcha {
				background-image: url($ajax_url);
			}
		</style>
EOT;
}

// Add admin-ajax endpoint
add_action('wp_ajax_spx_pixel', 'spx_pixel_submit');
add_action('wp_ajax_nopriv_spx_pixel', 'spx_pixel_submit');

function spx_pixel_submit() {
	$ip_address = $_SERVER['REMOTE_ADDR'];
	set_transient('spx_allow_' . $ip_address, '1', DAY_IN_SECONDS);
	wp_die();
}

// Check form submission
add_action('wpforms_process_before', 'spx_wpforms_process_before', 10, 2);

function spx_wpforms_process_before( $entry, $form_data ) {
	$ip_address = $_SERVER['REMOTE_ADDR'];
	if (false === get_transient('spx_allow_' . $ip_address)) {
		wp_die('I don\'t think so!');
	}
}

what exactly this code doing?

Do not accept form submission from who has not loaded our CSS background image.

Thanks for your answer!
Ho, I see, and how is it block spam?
If the user didn't focus on the form so basically it means that's a robot?
From your experience, how safe is that?

Have a read of this. It should answer your questions.

https://rosswintle.uk/2021/03/css-only-spam-prevention-allow-list-captcha/

Thank you!!!