说明：  了解WWW的概念 了解统一资源定位符（URL）的结构 了解什么是HTML语言 理解HTTP协议的请求／响应机制，了解协商与缓存的作用
(Understanding the concept of understanding WWW uniform resource locator (URL) to understand what is the structure of HTML language understanding HTTP protocol request/response mechanism, in consultation with the understanding of the role cache)