Simple way to add a Google reCAPTCHA v2. Full version on https://looons404.com/

index.html

<!DOCTYPE html>
<html lang="fr">
<head>
  <meta charset="UTF-8">
  <title>Google reCAPTCHA</title>
  <link rel="stylesheet" href="style.css">
  <script src="https://www.google.com/recaptcha/api.js"
    async defer>
  </script>
</head>
<body>
  <div class="g-recaptcha" data-sitekey="your-site_key_here"></div>
</body>
</html>