1. doctype What is the role?
<! DOCTYPE> declaration must be the first line of HTML documents located in <html> tag before.
<! DOCTYPE> declaration is not an HTML tag; it is an indication about which web browser HTML version of the page using the instructions written.
There are three <! DOCTYPE> declaration in HTML 4.01. Only one in HTML5: <! DOCTYPE HTML >
2.HTML, XHTML, XML What is the difference?