SOURCE

function appendChild (el, chd) {
    el.appendChild(chd)
}
var div = document.createElement('div', '', "z-index:999;background:rgba(0,0,0,.5),width:'100px',height:'100px'")
//创建iframe
var iframe = document.createElement('iframe','',"heigth:'100px',width:'100px'");
// document.appendChild(iframe);
// iframe.style.display = 'none';
iframe.style.height = '100px';
iframe.style.width = '100px';
iframe.src = 'www.baidu.com';
appendChild(div, iframe);
console.log(document.getElementsByTagName('div'))
document.getElementsByTagName('div')[0].appendChild(div)

<div></div>