On the HTML document you have often seen that there is a <!DOCTYPE html> declaration before the <html> tag. HTML <!DOCTYPE> tag is used to inform the browser about the version of HTML used in the document. It is called as the document type declaration (DTD).
Technically <!DOCTYPE > is not a tag/element, it just an instruction to the browser about the document type. It is a null element which does not contain the closing tag, and must not include any content within it.
Actually, there are many type of HTML e.g. HTML 4.01 Strict, HTML 4.01 Transitional, HTML 4.01 Frameset, XHTML 1.0 Strict, XHTML 1.0 Transitional, XHTML 1.0 Frameset, XHTML 1.1 etc.
The <!DOCTYPE> declaration refers Document Type Declaration (DTD) in HTML 4.01; because HTML 4.01 was based on SGML. But HTML 5 is not SGML based language.
The doctype declaration differs between HTML versions. The HTML 5 doctype declaration is given below.
Syntax:
<!DOCTYPE html>Following are some specifications about the HTML <!DOCTYPE>
| Display | None |
| Start tag/End tag | Start tag only |
| Usage | Structural |
Let’s see an example of HTML document with doctype declaration.
<!DOCTYPE html>
<html>
<head>
<title>This is the title</title>
</head>
<body>
This is the content of the document.
</body>
</html> Supporting Browsers
| Element |  Chrome |  IE |  Firefox |  Opera |  Safari |
| <!DOCTYPE> | Yes | Yes | Yes | Yes | Yes |