var headerBox = document.getElementById("header");
var p = document.createElement("p");
insertAfter(p,headerBox);

function insertAfter( newElement, targetElement ){ // newElement是要追加的元素 targetElement 是指定元素的位置
  var parent = targetElement.parentNode;
  if( parent.lastChild == targetElement ){
    parent.appendChild( newElement, targetElement );
  }else{
    parent.insertBefore( newElement, targetElement.nextSibling );
  };
};