Is it possible to create a site without a markup language?

Yes, but it does not make any sense. It does some and I even like the idea in web application, but for a website that needs to be used under universal web conditions, which needs indexing by search engines, it is not viable.

Load the JS file and it will bring everything for it, probably via AJAX, then the script will mount the DOM" in the hand ", that is, putting the content, formatting everything in the objects of the document. Instead of marking what you want, you will program what you want in an imperative way. As soon as you can access everything in the document with JS you can also manipulate it, you can create new nodes and go on the page.