https://lnikki.la/articles/friendly-useful-404-pages/

404.html

<a id="alert-twitter"
   href="https://twitter.com/intent/tweet?text=%40lnikkila+Hey%2C+this+link+is+dead%3A&amp;url=">
   Twitter link
</a>

<a id="alert-email"
   href="mailto:hello@lnikki.la?subject=Found+a+dead+link&amp;body=Hey%2C+this+link+is+dead%3A+">
   Email link
</a>

<script type="text/javascript" src="//platform.twitter.com/widgets.js"></script>
<script>
  (function() {
    var twitter = document.getElementById('alert-twitter');
    var email = document.getElementById('alert-email');

    twitter.href += encodeURIComponent(window.location);
    email.href += encodeURIComponent(window.location);
  })();
</script>

link-formats

https://twitter.com/intent/tweet?text=[@username and message]&url=[URL]

mailto:[email]?subject=[subject]&body=[message]