What is the importance of the HTML DOCTYPE?
The doctype declaration should be the very first thing in an HTML document, before the html tag.
The doctype declaration is not an HTML tag; it is an instruction to the web browser about what version of the markup language the page is written in.
The doctype declaration refers to a Document Type Definition (DTD). The DTD specifies the rules for the markup language, so that the browsers can render the content correctly.