It really depends on the context of the term 'headers'.
For example, complete headers of an email show the RFC information such as the values of the To, From, Subject, Content/Type, etc fields.
In PERL which generates HTML pages, complete headers refers to what is between the HTML <head></head> tags. At the very least, PERL generated HTML pages need to have the mime type set to text/html.
print "Content-type: text/html\n\n";
I hope this helps some.