/*
* @function 动态加载css文件
* @param {string} options.url -- css资源路径
* @param {function} options.callback -- 加载后回调函数
* @param {string} options.id -- link标签id
*/
function loadCss(options){
 var url = options.url,
 callback = typeof options.callback == "function" ? options.callback : function(){},
 id = options.id,
 node = document.createElement("link"),
 supportOnload = "onload" in node,
 isOldWebKit = +navigator.userAgent.replace(/.*(?:AppleWebKit|AndroidWebKit)\/?(\d+).*/i, "$1") < 536, // webkit旧内核做特殊处理
 protectNum = 300000; // 阈值10分钟，一秒钟执行pollCss 500次
 node.rel = "stylesheet";
 node.type = "text/css";
 node.href = url;
 if( typeof id !== "undefined" ){
 node.id = id;
 }
 document.getElementsByTagName("head")[0].appendChild(node);
 // for Old WebKit and Old Firefox
 if (isOldWebKit || !supportOnload) {
 // Begin after node insertion
 setTimeout(function() {
  pollCss(node, callback, 0);
 }, 1);
 return;
 }
 if(supportOnload){
 node.onload = onload;
 node.onerror = function() {
  // 加载失败(404)
  onload();
 }
 }else{
 node.onreadystatechange = function() {
  if (/loaded|complete/.test(node.readyState)) {
  onload();
  }
 }
 }
 function onload() {
 // 确保只跑一次下载操作
 node.onload = node.onerror = node.onreadystatechange = null;
 // 清空node引用，在低版本IE，不清除会造成内存泄露
 node = null;
 callback();
 }
 // 循环判断css是否已加载成功
 /*
 * @param node -- link节点
 * @param callback -- 回调函数
 * @param step -- 计步器，避免无限循环
 */
 function pollCss(node, callback, step){
 var sheet = node.sheet,
  isLoaded; 
 step += 1;
 // 保护，大于10分钟，则不再轮询
 if(step > protectNum){
  isLoaded = true;
  
  // 清空node引用
  node = null;
  callback();
  return;
 }
 if(isOldWebKit){
  // for WebKit < 536
  if(sheet){
  isLoaded = true;
  }
 }else if(sheet){
  // for Firefox < 9.0
  try{
  if(sheet.cssRules){
   isLoaded = true;
  }
  }catch(ex){
  // 火狐特殊版本，通过特定值获知是否下载成功
  // The value of `ex.name` is changed from "NS_ERROR_DOM_SECURITY_ERR"
  // to "SecurityError" since Firefox 13.0. But Firefox is less than 9.0
  // in here, So it is ok to just rely on "NS_ERROR_DOM_SECURITY_ERR"
  if(ex.name === "NS_ERROR_DOM_SECURITY_ERR"){
   isLoaded = true;
  }
  }
 }
 setTimeout(function() {
  if(isLoaded){
  // 延迟20ms是为了给下载的样式留够渲染的时间
  callback();
  }else{
  pollCss(node, callback, step);
  }
 }, 20);
 }
}

var url = options.url,
 callback = typeof options.callback == "function" ? options.callback : function(){},
 id = options.id,
 node = document.createElement("link");
node.rel = "stylesheet";
node.type = "text/css";
node.href = url;
if( typeof id !== "undefined" ){
 node.id = id;
}
document.getElementsByTagName("head")[0].appendChild(node);

try{
 if(sheet.cssRules){
 isLoaded = true;
 }
}catch(ex){
 // 火狐特殊版本，通过特定值获知是否下载成功
 // The value of `ex.name` is changed from "NS_ERROR_DOM_SECURITY_ERR"
 // to "SecurityError" since Firefox 13.0. But Firefox is less than 9.0
 // in here, So it is ok to just rely on "NS_ERROR_DOM_SECURITY_ERR"
 if(ex.name === "NS_ERROR_DOM_SECURITY_ERR"){
 isLoaded = true;
 }
}

var isOldWebKit = +navigator.userAgent.replace(/.*(?:AppleWebKit|AndroidWebKit)\/?(\d+).*/i, "$1") < 536; // webkit旧内核做特殊处理
if(isOldWebKit){
 // for WebKit < 536
 if(sheet){
 isLoaded = true;
 }
}

setTimeout(function() {
 if(isLoaded){
 // 延迟20ms是为了给下载的样式留够渲染的时间
 callback();
 }else{
 pollCss(node, callback, step);
 }
}, 20);

var protectNum = 300000, // 阈值10分钟，一秒钟执行pollCss 500次
 step = 0;
// 很多代码....
step += 1;
// 保护，大于10分钟，则不再轮询
if(step > protectNum){
 isLoaded = true;
 // 清空node引用
 node = null;
 callback();
 return;
}

// for Old WebKit and Old Firefox
if (isOldWebKit || !supportOnload) {
 // Begin after node insertion
 setTimeout(function() {
 pollCss(node, callback, 0);
 }, 1);
 return;
}

function onload() {
 // 确保只跑一次下载操作
 node.onload = node.onerror = node.onreadystatechange = null;
 // 清空node引用，在低版本IE，不清除会造成内存泄露
 node = null;
 callback();
}

if(supportOnload){
 node.onload = onload;
 node.onerror = function() {
 // 加载失败(404)
 onload();
 }
}

if(supportOnload){
 // 代码...
}else{
 node.onreadystatechange = function() {
 if (/loaded|complete/.test(node.readyState)) {
  onload();
 }
 }
}