Hi Gabriele,

Yes, I confirm you that a client built with the HTML library version 5 will work fine with the new Lightstreamer server 5.x Colosseo. Indeed, as a rule of thumb, newer Lightstreamer servers usually support older clients. Viceversa newer clients are usually not compatible with older servers.

However, the new JavaScript Client Library 6.0 is much easier to use, much faster, and has fully support of WebSocket protocol. So in order to get all this benefits we highly recommend upgrading to the latest version of the JavaScript client lib too.
Please refer here for a tutorial on how to Upgrade to JavaScript Client Library 6.