What is <!DOCTYPE> Tag?
This is not a html tag, this is just a declaration tag. The <!DOCTYPE> declaration refers Document Type Declaration (DTD).
Why <!DOCTYPE> Tag use?
This is not nesessor to use <!DOCTYPE> tag, but this is declar the html virson. So, you must be use this <!DOCTYPE> Tag. If not, perhaps the program will show you error some time.
How to use <!DOCTYPE> Tag?
You can write this type of value in <!DOCTYPE> because this is not case sensitive. The <!DOCTYPE> tag always use starting point in the html program.
<!DOCTYPE html>
<!DocType html>
<!Doctype html>
<!doctype html>
This is all currect.
Example:
<!DOCTYPE html>
<html>
<head>
<title> Your title here </title>
</head>
<body>
Write here your post body
</body>
</html>