添加和删除页面元素

 function Button1_onclick(){    var divone=document.getElementById("ModifyDiv");     if(!divone)    {        divone=document.createElement("Div");        divone.id="ModifyDiv";        divone.style.width="200px";        divone.style.height="180px";        divone.innerHTML="aaaaa";         document.getElementById("aa").appendChild(divone);    }}function Button2_onclick() {var divone=document.getElementById("ModifyDiv"); if(divone){divone.parentNode.removeChild(divone); } }function Button3_onclick(){alert(document.getElementById("ModifyDiv").innerHTML);}