Trying to understand headers HTTP

AJAX Crash Course:

https://www.youtube.com/watch?v=82hnvUYY6QA&t=429s&fref=gc&dti=743759285743274

Content Negotiation:
https://en.wikipedia.org/wiki/Content_negotiation

Now that I have that out of my system....I am looking at Content Negotiation, AJAX requests, and HTTP Headers...

Goal
(1) Accept different file types other than the standard html when hitting a URL (e.g, text/turtle, application+jsonld(?))
(2) Use X509 certificates in the HTTP request (understand authentication and authorization over HTTP)
I have no clue what the Headers should look like in this algorithm:

Maybe see: http://raptorlicious.blogspot.com/2017/09/i-want-cors-with-credentials-notes.html


(http://dig.csail.mit.edu/2009/Papers/SPOT/foaf-ssl-spot2009.pdf)

"The act of retrieving a representation of a resource identified by a URI is known as dereferencing that URI."
https://www.w3.org/2001/tag/doc/httpRange-14/2007-05-31/HttpRange-14

Also see: https://web-payments.org/specs/source/http-signatures/