说明：  XHTML标准参考手册,XHTML是The Extensible HyperText Markup Language可扩展标识语言的缩写。目前推荐遵循的是W3C于2000年1月26日推荐XML1.0（参考http://www.w3.org/TR/xhtml1）。XML虽然数据转换能力强大，完全可以替代HTML，但面对成千上万已有的站点，直接采用XML还为时过早。因此，我们在HTML4.0的基础上，用XML的规则对其进行扩展，得到了XHTML。简单的说，建立XHTML的目的就是实现HTML向XML的过渡。
(XHTML standard reference manual, XHTML is The Extensible HyperText Markup Language can be an acronym for Extensible Markup Language. At present, it is recommended to follow the W3C in January 26, 2000 Recommended XML1.0 (reference http://www.w3.org/TR/xhtml1). Although the ability of XML data conversion powerful, can replace HTML, but in the face of tens of thousands of existing sites, direct use of XML is still too early. Therefore, we HTML4.0 based on XML used the rules of their expansion has been XHTML. Simply put, the purpose of the establishment of XHTML is to realize the transition of HTML to XML.)