The QNX Neutrino architecture is designed to be both simple and powerful. A powerful benefit is the use of sandboxing, so that applications can run in complete isolation from each other. The platform provides a multiprocess architecture that allows system developers to partition the UI into a set of core and sandboxed apps. With this architecture, multiple WebViews (or windows) can either share a common engine instance or run in their own engine instance. Each WebView (equivalent to a tab in a desktop browser) can be implemented with a separate (and different) JavaScript application framework (for example, jQuery Mobile or Sencha Touch).
Applications running within the same instance of the HTML5 engine wouldn't be isolated from each other. Incorrect behavior in one application could impact all other applications. For example, a hung or stalled JavaScript thread in one application would hang all other HTML5 applications. This is why the QNX Neutrino architecture runs each application in its own private HTML5 engine instance (its "sandbox"). This design isolates applications so that any problems they encounter don't impact other applications. The sandbox model does, however, increase the system's memory footprint.