How does one get started with web development for mobile devices (including phones)? I know some old good HTML & JS, but I'm wondering what the modern way is.

To make it concrete, I need to implement a client for an enterprise app. Think of a login-protected dashboard for managers.

Given that I only have basic knowledge and have never implemented such applications, what do I need to take into account when choosing platform and technology?

1. Shall I go for Android / iPhone, or HTML & JS, or HTML5? What are the general pros and cons that I need to consider?
2. What resources (preferably online) do you recommend for learning the rudiments? I mean especially HTML & JS: What does the site need to be like to work great on smartphones (and not only on desktops)? I mean possible UI glitches, limited support for some HTML/JS features, page size, and whatever other important details are there.