function toggle(element) {
    if (document.getElementById(element).style.display == "none") {
    document.getElementById(element).style.display = "";


    } else {
    document.getElementById(element).style.display = "none";

    }
    }


    <a href="javascript:toggle('hidden')">Open spoiler</a>
    <div id="hidden" style="display:none;">
    <iframe    ></iframe>
    </div>

iframe{
  display: none;
}

var yourElement = document.getElementById('elementId');
yourElement.addEventListener('load', function(){
  document.getElementById('iframeId').display = 'inline';
}, false)