Some guys could say it is easy, but others could say it is beyond their knowledge of system administrators. Nethgui was easy, but you could do only what nethgui allowed you, cockpit let your free hands, however you have to write everything.
From what I think, vanilla js is not the way to go, vuejs is better, even if a js knowledge is needed, the data binding is easier. Vuejs takes care to update in the dom, the vars you have declared.
I started a scarfold of a rpm for cockpit vuejs, it is a starting point, after that you have to write your html/js/css but now you have several hundred thousand lines of code as example in the nethserver github repository.
If you only want to display a web application (or a nethgui panel) I did another way