Edit: some more public details
The default browser on the BlackBerry 10 platform was a completely new browser application. The chrome was written in HTML, CSS and JavaScript. Being able to develop the chrome on your desktop browser or being able to run inspector remotely and using your desktop was very handy. The core was a command line application called webplatform that "launched" a url that was a "webapp". The webapp had API's exposed to it such as creating WebView's in or out of process (yup blackberry has had multi-process tabs for a while now...). One joy was being able to pull up the Javascript console for the browser WebView and dynamically calling exposed c++ API's in any WebView in any process to test out features or diagnose problems.
It started out as a quick little proof of concept I tossed together over a day and the upside was large enough to invest time into. One of the reasons for making the main browser in html was that as a platform we wanted web application to succeed. Eating our own dogfood we made sure webapps could handle the job. The API's that were needed were there, memory usage were low, startup time was fast etc. And if you search for reviews of the blackberry 10 browser you will find that the end result was a success.
Edit 2: Much more in depth information can be found on this video which was a presentation given at Blackberry Jam by several of my colleagues. Skip to the 23 minute mark to see some actual code of what a webapp browser would look like.