Chrome的Waterfall

参考：https://developers.google.com/web/tools/chrome-devtools/network-performance/reference#timing

浏览器根据html中外连资源出现的顺序，依次放入队列（Queue）,然后根据优先级确定向服务器获取资源的顺序。同优先级的资源根据html中出现的先后顺序来向服务器获取资源

• Queueing. 出现下面的情况时，浏览器会把当前请求放入队列中进行排队
  • 有更高优先级的请求时.
  • 和目标服务器已经建立了6个TCP连接（最多6个，适用于HTTP/1.0和HTTP/1.1）
  • 浏览器正在硬盘缓存上简单的分配空间
• Stalled. 请求会因为上面的任一个原因而阻塞
• DNS Lookup. 浏览器起正在解析IP地址.
• Proxy negotiation. The browser is negotiating the request with a proxy server.
• Request sent. The request is being sent.
• ServiceWorker Preparation. The browser is starting up the service worker.
• Request to ServiceWorker. The request is being sent to the service worker.
• Waiting (TTFB). 浏览器等待响应第一个字节到达的时间. 包含来回的延迟时间和服务器准备响应的时间.
• Content Download. The browser is receiving the response.
• Receiving Push. The browser is receiving data for this response via HTTP/2 Server Push.
• Reading Push. The browser is reading the local data previously received.

如图所示，select2_metro.css在位置上要比avatar1_small.png和index.js靠后，但是优先级确实最高(Higthest-->High-->Medium-->Low),所以这个下载顺序是：select2_metro.css-->index.js-->avatar1_small.png