HTML <!doctype> Tag
<!doctype> tag is used for specifying which language and version the document is using. This is referred to as the document type declaration (DTD).
Technically speaking, the
<!doctype> tag does not represent an HTML element (there is no
<!doctype> element). However, it has been included here, as it is a required preamble to all HTML documents.
Also, it is usually referred to as the "doctype declaration" as opposed to the "doctype tag". Having said that, "tag" is a pretty loose term that tends to be applied quite generically across the web, so I've included it in the list of "HTML tags".
<!doctype> declaration is written as
<!doctype> (no end tag) with the language being specified inserted within the tag. The
<!doctype> declaration is case-insensitive (i.e. can be either uppercase or lowercase).
<!doctype> declaration must go right at the top of the page, before any other HTML code.