Doctypes - short for 'document type' - help browsers to understand the version of HTML the document is written in for better interpretability. Doctype declarations are not HTML tags and belong at the very top of a document. This topic explains the structure and declaration of various doctypes in HTML.
<!DOCTYPE> declaration is not an HTML tag. It is used for specifying which version of HTML the document is using. This is referred to as the document type declaration (DTD).
<!DOCTYPE> declaration is NOT case sensitive. To check if the HTML of your Web pages is valid, go to W3C's validation service.