The HTML <doctype> tag is used for specifying which version of HTML the document is using. This is referred to as the document type declaration (DTD).
The <!DOCTYPE> declaration is the first thing you can see in the HTML document. It gets added before the <html> tag. The <!DOCTYPE> tag does not have an end tag!.
HTML 4.01 had 3 possible doctypes − HTML 4 Strict, HTML 4 Transitional, and HTML 4 Frameset. Every HTML document you create should have one of these three DTDs.
This document type includes all HTML elements except those that have been deprecated and those that appear in frameset documents.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
This document type includes all HTML elements including those that have been deprecated.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">;
This document type includes all HTML elements in the transitional DTD as well as those in a framed document.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
In HTML5 there is only one declaration i.e.