通过原生js添加div和css

function createStyle(){
    return"*{padding:0;margin:0;border:0}.loading{640px;height:1024px;margin:0 auto}.loadImg{640px;height:1024px;animation:fadeInOut 3s linear;-webkit-animation:fadeInOut 3s linear;-moz-animation:fadeInOut 3s linear;opacity:0}@keyframes fadeInOut{0%{opacity:0}25%{opacity:.5}45%{opacity:1}50%{opacity:1}55%{opacity:1}75%{opacity:.5}100%{opacity:0}}@-moz-keyframes fadeInOut{0%{opacity:0}25%{opacity:.5}45%{opacity:1}50%{opacity:1}55%{opacity:1}75%{opacity:.5}100%{opacity:0}}@-webkit-keyframes fadeInOut{0%{opacity:0}25%{opacity:.5}45%{opacity:1}50%{opacity:1}55%{opacity:1}75%{opacity:.5}100%{opacity:0}}"
}
function addStyle(){
    var style = document.createElement("style");
    style.type = "text/css";
    style.innerHTML = createStyle();
    window.document.head.appendChild(style);
}
function addDom(){
    var div = document.createElement("div");
    div.className = "loading";
    window.document.body.appendChild(div);
    var img = document.createElement("img");
    img.src="img/bg.png";
    img.id="loadImg";
    img.className="loadImg";
    div.appendChild(img);
}
window.onload=function(){
    addStyle();
    addDom();
}