How to solve the problem with fake coordinates in browser multiplayer?

Question

Game development

Hello! There is an idea - to make a multiplayer game. My task is to make sure that 2 players see each other's movements on their monitors. I want to implement this by sending JSON with each player's coordinates to each other. Graphics will be implemented in HTML and CSS. Each player will have their own file on the webserver, which will manage everything (yes, I know that this is not the best way to do it, but purely for practical purposes, I want to do it this way, although in the future, of course, I will change this)
Problem here in what - each player can open the console and change their coordinates to any other, which will change the general JSON, so the other player will see all these magical movements.
How to protect yourself from this? Thank you in advance.