javascript add div to body with class

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);// correct me if im wrong
var MainDiv = document.createElement('div');
var ChildDiv1 = MainDiv.createElement('div');
ChildDiv1.className = 'contextmenu_contextMenu_3_a2Z contextmenu_ContextMenuFocusContainer_2thYU';
ChildDiv1.tabIndex = "0";
ChildDiv1.style = "visibility: visible; bottom: 56px; right: 68px;"
var ChildDiv2 = ChildDiv1.createElement('div');
ChildDiv1.className = 'contextmenu_contextMenuContents_1yyTu';
var DivButton1 = ChildDiv2.createElement('div');
DivButton1.className = 'contextmenu_contextMenuItem_CBC-y contextMenuItem';
DivButton1.value = "Upload an Image";
var DivButton2 = ChildDiv2.createElement('div');
DivButton2.className = 'contextmenu_contextMenuItem_CBC-y contextMenuItem';
DivButton2.value = "Share Your Trade Offer URL";
document.body.appendChild(MainDiv);