JavaScript implements getting remote html to the current page

<p id="includeHtml"></p>

function clientSideInclude(id, url) {
  var req = false;
  // Safari, Firefox, 及其他非微软浏览器
  if (window.XMLHttpRequest) {
    try {
      req = new XMLHttpRequest();
    } catch (e) {
      req = false;
    }
  } else if (window.ActiveXObject) {

    // For Internet Explorer on Windows
    try {
      req = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
      try {
        req = new ActiveXObject("Microsoft.XMLHTTP");
      } catch (e) {
        req = false;
      }
    }
  }
  var element = document.getElementById(id);
  if (!element) {
    alert("函数clientSideInclude无法找到id " + id + "。" +
      "你的网页中必须有一个含有这个id的p 或 span 标签。");
    return;
  }
  if (req) {
    // 同步请求，等待收到全部内容
    req.open('GET', url, false);
    req.send(null);
    if (req.status == 404) {
      clientSideInclude(id, 'error.html')
    } else {
      element.innerHTML = req.responseText;
    }
  } else {
    element.innerHTML =
      "对不起，你的浏览器不支持" +
      "XMLHTTPRequest 对象。这个网页的显示要求" +
      "Internet Explorer 5 以上版本, " +
      "或 Firefox 或 Safari 浏览器，也可能会有其他可兼容的浏览器存在。";
  }
}

clientSideInclude(includeHtml, "http://XXXXX.html");//页面中的一个p的id为includeHtml