...确、一致地显示，就得告诉网页浏览器你正应用HTML或者XHTML的哪个版本，包含一个网页开头处的文档类型声明(doctype declaration)是什么意思。
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The DOCTYPE declaration should always be the first line in an XHTML document.
文档声明必须出现在XHTML文档的首行。
Your pages must have a DOCTYPE declaration if you want them to validate as correct XHTML.
如果你想使你的页面满足正确的XHTML的有效性你就必须使用DOCTYPE声明。
Remember, you can use any of the XHTML document types as long as you include the full !DOCTYPE declaration.
请记住，只要包含全部 ! DOCTYPE 声明，就可以使用任何 XHTML 文档类型。
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