<! Doctype> about the first line of an HTML document

DOCTYPE is a shorthand for document type, which is an essential key component of the DOCTYPE declaration to create a page that conforms to the standard.

2017-05-20

label definition and usage instructions

2. <! The doctype> declaration is located in the first position in the document, before the declaring the document's parsing type (document.compatmode), avoiding the browser's bizarre pattern;

   1 <!  DOCTYPE html> 2 

4. <! The doctype> declaration is not an HTML tag; it is used to tell the Web browser which HTML version the page is using;

6. In HTML 4.01, <! The doctype> declaration requires a reference to a DTD (document type declaration) because HTML 4.01 is based on SGML (Standard generalized Markup Language-generic Markup language). The DTD specifies the rules of the markup language to ensure that the browser renders the content correctly;

8. HTML5 is not based on SGML and therefore does not require a reference DTD;

10. <! The doctype> declaration does not have an end tag and is not case sensitive.

Css1compat: Standard mode, browser using the standard resolution rendering page; Backcompat: Weird mode, the browser uses its own weird mode to parse the rendered page.

Common DOCTYPE declarations

HTML 5

<! DOCTYPE HTML >

There are 3 types of DTD (document type definition) declarations in HTML4.01 that can be selected: transitional (Transitional), Strict (Strict), and Frame (Frameset).

HTML4.01 Transitional

<! DOCTYPE HTML PUBLIC "-//w3c//dtd HTML 4.01 transitional//en" "Http://www.w3.org/TR/html4/loose.dtd " >

HTML4.01 Strict

<! DOCTYPE HTML PUBLIC "-//w3c//dtd HTML 4.01//en" "HTTP://WWW.W3.ORG/TR/HTML4/STRICT.DTD" >

HTML4.01 Frameset

<! DOCTYPE HTML PUBLIC "-//w3c//dtd HTML 4.01 frameset//en" "Http://www.w3.org/TR/html4/frameset.dtd " >

Reference: More

<! Doctype> about the first line of an HTML document