single HTML (structure)
single JS (events and callbacks)
single CSS with media queries to adapt to various devices (e.g., rearrange elements layout, hide/show elements that would not be critical on an smaller device)
single server software

>So, you're saying that using HTML/CSS, I can create a single app that looks and behaves like a native app on iPhone, Android (in it's various flavors), and Windows Phone simply by implementing media queries?
>
>
>>CSS3 media queries allow that
>>http://www.w3.org/TR/css3-mediaqueries/