JavaScript for basic CSS fixed overlay centered on page

gistfile1.js

var d = document.createElement("div");
d.style.background = "#FFFF00";
d.style.top = 0;
d.style.position = "fixed";
d.style.top = "50%";
d.style.left = "50%";
d.style.marginLeft = "-150px";
d.style.zIndex = "100000";
d.style.width = "300px";
var t = document.createTextNode("Message goes here...");
d.appendChild(t);
document.body.appendChild(d);