No need for Socket.io, Mongo, Express, or Node.js server, ChatEngine works with serverless technology. The HTML, CSS, and JavaScript code here is used in the basic chat app tutorial on the PubNub blog.
Chat App Demo - Works on Mobile and Desktop
In order to get this working in your own instance, you must sign up for a free PubNub account and hit the "Deploy ChatEngine" button. This will give you free API keys for the serverless app. Use the form in this tutorial to get going in 5 minutes: Alternative to Building a Socket.IO Chat App