Html4.0.1,h5,xml, Document Declaration differences

<! 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.

Tip: Always add <! to your HTML document Doctype> declaration so that the browser can learn the document type.

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.

HTML5 is not based on SGML, so you do not need to reference a DTD.

There are three types of <! in HTML 4.01 Doctype> statement. There is only one of the HTML5:

Html4.0.1,h5,xml, Document Declaration differences