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Jun 24, 2016 am 11:20 AM

声明必须是 HTML 文档的第一行，位于标签之前。

声明不是 HTML 标签；它是指示 web 浏览器关于页面使用哪个 HTML 版本进行编写的指令。

用来声明使用的html版本。

在 HTML 4.01 中，声明引用 DTD，因为 HTML 4.01 基于 SGML。DTD 规定了标记语言的规则，这样浏览器才能正确地呈现内容。

HTML5 不基于 SGML，所以不需要引用 DTD。

提示：请始终向 HTML 文档添加声明，这样浏览器才能获知文档类型。

在doctype中，html4.01中有三种，在h5中只有一种。

“文档类型声明”的英文译文是 doctype declaration。

html5:

<!DOCTYPE html>

html4.01 Transitional 该 DTD 包含所有 HTML 元素和属性，包括展示性的和弃用的元素（比如 font）。不允许框架集（Framesets）。

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

html4.01 Frameset 该 DTD 等同于 HTML 4.01 Transitional，但允许框架集内容。*J:允许使用frameset

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">

html4.01 Script 该 DTD 包含所有 HTML 元素和属性，包括展示性的和弃用的元素（比如 font）。不允许框架集（Framesets）。

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

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