HTML自身是一个标记语言，XML则提供了一个定义标记语言的框架
XML 用于描述数据。
HTML是用于显示数据。
XML是可以扩展的，标签可以自定义。
HTML不区分大小写，xml区分大小写
HTML is a markup language itself while XML provides a framework for defining markup languages.
XML was designed to describe data.
HTML was designed to display data.
XML is extensible.
HTML is case insensitive while XML is case sensitive.