Where does the backend end and the frontend begin?

Question

Backend

I have never dealt with web development.
The task is to order a site with a rather specific functionality. In such a way that all specific functionality (algorithms) is implemented separately, and the entire design is subsequently implemented. (I accept design=frontend).
In my view, the front-ender can be provided with bare forms with functionality, like in this picture, for example, boring buttons and fields in the web 1.0 style, and he can already write beautiful pop-up menus, a color scheme, arrange everything beautifully, for example, change boring square buttons to flat ones in windows 8 style and the like, make a great user interface on top of the finished algorithm.
What are the mistakes in this approach? Do I need to plan front-end features in advance during the development of the back-end?
*considering that you don’t SPECIFICALLY know what is required from the design

Answer 1

For starters, design != frontend.
Specifically for your situation, a lot depends on what the frontend will be written on. If you write it on frameworks, then it is better with a ready-made design, otherwise you will have to do double work. And for a painless separation of the frontend and backend, they usually make an API. In general, to begin with, decide whether you will have a "thin" client and a "thick" server, or vice versa.