URL及HTTP请求过程

看过老外的一篇文章《What really happens when you navigate to a URL》写的很好，通俗易懂。这对网站的规划、前端优化，都有很强的指导意义。

节选一点：
As a software developer, you certainly have a high-level picture of how web apps work and what kinds of technologies are involved: the browser, HTTP, HTML, web server, request handlers, and so on.

In this article, we will take a deeper look at the sequence of events that take place when you visit a URL.

1. You enter a URL into the browser
2. The browser looks up the IP address for the domain name
3. The browser sends a HTTP request to the web server
4. The facebook server responds with a permanent redirect
5. The browser follows the redirect
6. The server ‘handles’ the request
7. The server sends back a HTML response
8. The browser begins rendering the HTML
9. The browser sends requests for objects embedded in HTML
10. The browser sends further asynchronous (AJAX) requests

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