Why does html5 only need to write doctype?

Because html5 is not based on SGML (Standard General Markup Language), there is no need to reference the DTD, but doctype is needed to standardize the behavior of the browser, that is, to run in the normal way, so html5 only needs to write Just doctype. "!DOCTYPE" is a document type declaration in the Standard Universal Markup Language that tells the browser the version of markup used to write the page.

The operating environment of this tutorial: Windows 10 system, HTML5 version, Dell G3 computer.

Why do you only need to write doctype in html5? The browser runs as they should)

And HTML4.01 is based on SGML, so a reference to the DTD is needed to tell the browser the document type used by the document.

!DOCTYPE does not belong to the HTML (an application under the Standard Universal Markup Language) tag. It is a document type declaration of the Standard Universal Markup Language. In HTML, it tells the browser the version of the markup used to write the page.

Thedeclaration must be on the first line in the HTML5 document, before thetag. This tag tells the browser the HTML specification used by the document.

It is very important to specify !DOCTYPE in all HTML documents so that the browser understands the expected document type.

Examples are as follows:

HTML5 document with the correct document type (!DOCTYPE):

    
 文档的内容 

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