To further elaborate, the web-client is just that; a client. It doesn't contain the required facilities to interpret dmb files, which means it can't run code itself, which means it can't run or host the game.

All it can do is join running worlds because it's designed to interpret server messages, which is a massively less complicated task than trying to build a new dmb interpreter from the ground up.

I mean, yes. You could make a game single player, in the aspect that you can make it so players have no interaction with each other. Could place them each in a separate 'room' or use client images to keep players apart, keep chats out, ect.