What is an efficient way I can go about sending low latency data between users using html5, JavaScript and a server?
I have no experience with node.js, web sockets or other low latency software like that. It will need to be supported by the default browser in android and ios.
The WebSocket API is the best (read "lowest latency") solution native to browsers at the moment. Libraries like Socket.IO and whatnot can help you implement a WebSocket system with relative ease.
Eventually, you might want to consider the WebRTC Data Channel API as well. It's still very much in the early stages of being implemented across browsers, but with WebRTC, you can establish a direct peer-to-peer connection between browsers, which is even better than the WebSocket API, which requires a server between clients.
For low latency, you should send UDP packets directly to the user with Java DatagramSocket, but you can't do that from a browser, so you have to go through the server. You have to send the data to the server, queue it on the server, and have the receiver's browser periodically poll the server to check for any data.