It can be tricky to achieve especially when you actually want to have the game playable at a larger resolution. I'm not familiar with (modern) GameMaker, but possibly you can lock the resolution at which your game renders to 84x48, then have that output be scaled up in the browser. In Unity, I have the game's camera output to a render texture of that exact resolution, then scale it up x10. I'm sure there's other GameMaker users in the jam that could share their solution!