html-код doctype
<!DOCTYPE html>
This tag shows that it is running HTML5
тег doctype
<!DOCTYPE html> <!--This doesn't have closing tag like others-->
this means this is the version 5 of HTML or HTML5
html-код doctype
<!DOCTYPE html>
it lets the browser know about the version of HTML you
are using.but it is not nessesary
since your browser will render it anyways