I am creating a HTML5 web app for mobile devices and was asked to hide the browser nav bar (the back & forward buttons) (typo here prev.). How can I achieve
If you can use JQuery in your web-app than I would suggest you to go for NiceScroll plugin.
It can be used for both mobile and desktop browsers and will hide the browser's scrollbars. If your code is going beyond the viewport height of browser than it will make a custom scrollbar which will fadeout if not in use.
Here is its Demo.
Edit:
As per your update, I would like to add that I am actually not a native mobile web-app developer but while searching for your problems I found some SO questions that can help you to lead the way further:
And these tutorials: