DTD（document type definition，文档类型定义）是一系列的语法规则， 用来定义XML或(X)HTML的文件类型。浏览器会使用它来判断文档类型， 决定使用何种协议来解析，以及切换浏览器模式。
事实上DTD可以定义所有 SGML语族的文档类型，但由于太过繁琐， XML Schema反而更加流行。
多数HTML编辑器都会为我们添加一行DOCTYPE声明，但DOCTYPE却是我们最容易忽略的部分。 下面我们会看到，DOCTYPE声明 并不是可有可无的。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
DOCTYPE
DOCTYPE是用来声明文档类型和DTD规范的，一个主要的用途便是文件的合法性验证。 如果文件代码不合法，那么浏览器解析时便会出一些差错。HTML编辑器通常也会在语法高亮的同时提供合法性验证。
DOCTYPE声明包括标准版本和一个DTD文件的URI。常用的DOCTYPE声明有以下几种：
以下代码来自 http://www.w3school.com.cn/tags/tag_doctype.asp
HTML 5
<!DOCTYPE html>
HTML 4.01 Strict
该 DTD 包含所有HTML 元素和属性，但不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">
HTML 4.01 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">
浏览器模式
为了能够很好地显示满足标准的页面，又能最大程度兼容不合法的HTML。 浏览器厂商一般会提供两种浏览器模式：
- 标准模式（standards mode）：浏览器根据标准规约来渲染页面。
- 混杂模式（quirks mode）：浏览器采用更加宽松的、向后兼容的方式来渲染页面。
混杂模式下，浏览器会模仿旧浏览器的行为，比如IE6，在此基础上兼容新的标准特性。 混杂模式又称兼容模式、怪异模式等。
DOCTYPE切换
浏览器根据不同的DOCTYPE选择不同的渲染方法就叫做 DOCTYPE切换。 其实DOCTYPE切换就是用来识别和兼容旧网页的。
以下情况浏览器会采用标准模式渲染：
- 给出了完整的DOCTYPE声明
- DOCTYPE声明了Strict DTD
- DOCTYPE声明了Transitional DTD和URI
以下情况浏览器会采用混杂模式渲染：
- DOCTYPE声明了Transitional DTD但未给出URI
- DOCTYPE声明不合法
- 未给出DOCTYPE声明
如果你是使用最新标准编写的页面但未给出DOCTYPE声明，这时就可能会出现一些怪异的行为。 例如盒模型不正确、窗口的 size不正确等问题。所以，尽量为你网站的所有页面都给出合法的DOCTYPE声明。
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