This html tag should appear at the top of every valid html document.This tag tells the browser that what version of
html used to write html document.
Types od this type are as follows:
!DOCTYPE HTML PUBLIC "-//w3c// DTD html 4.01 //EN" "http://www.w3c.org/TR/html4/strict.dtd"
// This type of DTD includes all html elements & attributes but not deprecated elements & frameset are not allowed
!DOCTYPE HTML PUBLIC "-//w3c// DTD Transitional html 4.01//EN" "/http://www.w3c.org/TR/html4/transitional.dtd"
//This is DTD includes all html elements & attributes even deprecated elements but not framesets
!DOCTYPE HTML PUBLIC "-//w3c// Frameset 4.01//EN" "http://www.w3c.org/TR/html4/framseset.dtd"
// It contains all elements & attributes even deprecated & frameset also allowed
!DOCTYPE HTML PUBLIC "-//w3c//DTD XHTML 1.0 Strict//EN" "http://www.w3c.org/TR/xhtml1/DTD/xhtml1 - strict.dtd"
// It is like strict & markup must also be written as well-formed XML
!DOCTYPE HTML PUBLIC "-//w3c//DTD XHTML 1.0 Transitional//EN" "http://www.w3c.org/TR/xhtml1/DTD/xhtml1 - transitional.dtd"
// It is like transtional & markup must also be written as well-formed XML
!DOCTYPE HTML PUBLIC "-//w3c//DTD XHTML 1.0 Frameset//EN" "http://www.w3c.org/TR/xhtml1/DTD/xhtml1 - frameset.dtd"
// It is like framset & markup must also be written as well-formed XML
