Commonly the first line of code in an HTML or XHTML page is known as the DOCTYPE declaration that specifies the document type for the file. It's not a tag or component but rather a mechanism for telling browsers what edition of HTML or another markup language has been used to create the current page. Each HTML file needs a declaration of its document type to work properly.  Example:  < !DOCTYPE html >
Example
This is a Heading
This is HTML5 Doctype