To write web applications you must decide first if you are going on server side style or your application is splitted in frontend and backend.
In the first case every http handler you made return a template populated with your data (from databases or whatever). The templates are your frontend and should include JS and CSS libraries and frameworks (eg, AngularJS, Vue, Bootstrap,etc).
In the second case http handlers are simple endpoints for other frontend applications (written in Angular,React, etc) and return data in well known data exchange formats (like JSON).
In the both cases is better to use a framework (eg. gorilla toolkit to avoid reinventing the wheel for routes and subroutes, middlewares, cookies, sessions, etc.
However, take a look over this server side application skeleton that I wrote a while ago, maybe it helps.