There are many different documents in the Web world. The browser can display the document correctly only if you understand the type of document.
HTML also has a number of different versions, only the exact HTML version used in the page is fully understood, and the browser can display the HTML page exactly. This is <!. The use of doctype>.
<! 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. is a document type declaration of a standard generic markup language that is intended to tell the standard generic subscript language parser What document type definitions should be used to parse the document.
<! The doctype> declaration must precede the
<! The doctype> declaration is not case sensitive and has no end tag.
DOCTYPE declaration for HTML 5: <! DOCTYPE html>.
Sample code content:
<! DOCTYPE html>
<meta charset= "UTF-8" >
<title> Add a document description </title>
<body>
<p> Add a document description to the page </p>
</body>
Citation: http://www.mayacoder.com/lesson/lesson?lesson_id=27&knowledge_id=5
HTML&CSS Basic Learning Note 1.4-defining document types