新增加的HTTP状态码 -- 103

IETF公布了新的HTTP状态码-103， 总结一下就是提前预加载(css、js)文档，提升用户的访问速度。

Summary: a new status code that lets the server send headers early, before the main headers. This helps with optimisations like preloading. Example from the document:

      HTTP/1.1  Early Hints
      Link: </main.css>; rel=preload; as=style

      HTTP/1.1  Early Hints
      Link: </style.css>; rel=preload; as=style
      Link: </script.js>; rel=preload; as=script

      HTTP/1.1  OK
      Date: Fri,  May  :: GMT
      Content-Length:
      Content-Type: text/html; charset=utf-
      Link: </main.css>; rel=preload; as=style
      Link: </newstyle.css>; rel=preload; as=style
      Link: </script.js>; rel=preload; as=script

The client can start preloading the CSS and JavaScript before the main headers arrive. This is a nice optimisation. 
There are various security risks with sending multiple headers to non-conforming clients hence: “Therefore, a server might refrain from sending Early Hints over HTTP/1.1 unless the client is known to handle informational responses correctly.”

在头部信息到达之前，用户可以开始预加载CSS和JavaScript文件，这是一个很好的优化。 
因此，向不合格的客户机发送多个标头存在各种各样的安全风险:“因此，如果客户不知道正确处理信息响应，服务器可能会避免发送HTTP / 1.1的早期提示。”

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