Doctype tag: Doctype is the first line in your HTML Document. Doctype refers to the DTD(Document Type Declaration) tells about the document and its version which we are using.
In HTML 4.01 Strict the DTD can be declared as
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "www.w3.org/TR/html4/strict.dtd">
In HTML 4.01 Transitional the DTD can be declared as
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "www.w3.org/TR/html4/loose.dtd">
The doctype declaration is not case sensitive. In HTML5, DTD can be written as follows:
<!DOCTYPE html> <!doctype html> <!Doctype Html>