The importance of HTML DOCTYPE

definition and usage

<! The doctype> declaration must be the first line in the HTML document, before the

<! The doctype> declaration is not an HTML tag; It is a directive that instructs the Web browser to write which HTML version of the page to use .

In HTML 4.01, <! The doctype> declaration references a DTD because HTML 4.01 is based on SGML. The DTD prescribes the rules of the markup language so that the browser renders the content correctly.

3 Types of DOCTYPE

HTML 4.01 Strict

The DTD contains all the HTML elements and attributes, but does not include the display and deprecated elements (such as font). Frameset (framesets) is not allowed.

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

HTML 4.01 Transitional

The DTD contains all the HTML elements and attributes, including both display and deprecated elements (such as font). Frameset (framesets) is not allowed.

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

HTML 4.01 Frameset

The DTD is equivalent to the HTML 4.01 transitional, but allows the frameset content.

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

If the HTML does not add doctype, there may be a bug in different browsers. So HTML plus DOCTYPE is crucial.

Reference: http://www.w3school.com.cn/tags/tag_doctype.asp