doctypes is one of the important thing of each web-page, when you define a doctypes in the webpage then its tell the browser about the rules of the markup language, and as per the rules browser render the content of the web page.
Document Type Definition (DTD) is one of the most important part in html, doctypes is not the html tags or html elements but its define in the starting of the html web-page, so when any html web-page start execution then its first search for the doctypes and refer the rules as per the doctypes. once its done then start rendering of the webpage content.
Below is the list of different types of doctypes
1) HTML 4.01 Strict : <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> 2) HTML 4.01 Transitional : <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 3) HTML 4.01 Frameset : <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">