Why do we use HTML <!doctype> Tag
The HTML <!doctype> tag is used for specifying which language and version the document is using. This is referred to as the document type declaration (DTD).
The <!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).
The <!doctype> declaration must go right at the top of the page, before any other HTML code.