Adding div element to body or document in JavaScript

Using Javascript

var elemDiv = document.createElement('div');
elemDiv.style.cssText = 'position:absolute;width:100%;height:100%;opacity:0.3;z-index:100;background:#000;';
document.body.appendChild(elemDiv);

Using jQuery

$('body').append('<div style="position:absolute;width:100%;height:100%;opacity:0.3;z-index:100;background:#000;"></div>');

Try this out:-

http://jsfiddle.net/adiioo7/vmfbA/

Use

document.body.innerHTML += '<div style="position:absolute;width:100%;height:100%;opacity:0.3;z-index:100;background:#000;"></div>';

instead of

document.body.innerHTML = '<div style="position:absolute;width:100%;height:100%;opacity:0.3;z-index:100;background:#000;"></div>';

Edit:- Ideally you should use body.appendChild method instead of changing the innerHTML

var elem = document.createElement('div');
elem.style.cssText = 'position:absolute;width:100%;height:100%;opacity:0.3;z-index:100;background:#000';
document.body.appendChild(elem);

Instead of replacing everything with innerHTML try:

document.body.appendChild(myExtraNode);

improving the post of @Peter T, by gathering all solutions together at one place.

Element.insertAdjacentHTML()

function myFunction() {
    window.document.body.insertAdjacentHTML( 'afterbegin', '<div id="myID" style="color:blue;"> With some data...</div>' );
}
function addElement(){
    var elemDiv = document.createElement('div');
    elemDiv.style.cssText = 'width:100%;height:10%;background:rgb(192,192,192);';
    elemDiv.innerHTML = 'Added element with some data'; 
    window.document.body.insertBefore(elemDiv, window.document.body.firstChild);
    // document.body.appendChild(elemDiv); // appends last of that element
}
function addCSS() {
    window.document.getElementsByTagName("style")[0].innerHTML += ".mycss {text-align:center}";
}

Using XPath find the position of the Element in the DOM Tree and insert the specified text at a specified position to an XPath_Element. try this code over browser console.

function insertHTML_ByXPath( xpath, position, newElement) {
    var element = document.evaluate(xpath, window.document, null, 9, null ).singleNodeValue;
    element.insertAdjacentHTML(position, newElement);
    element.style='border:3px solid orange';
}

var xpath_DOMElement = '//*[@id="answer-33669996"]';
var childHTML = '<div id="Yash">Hi My name is <B>\"YASHWANTH\"</B></div>';
var position = 'beforeend';
insertHTML_ByXPath(xpath_DOMElement, position, childHTML);