The Short answer: Yes, you can develop apps using HTML / CSS / Javascript. Take two WebView Tutorials and call me in the morning.

The Long Answer: If you want to write apps for Android that use HTML / CSS / Javascript, you'll have to at least create a native WebView wrapper. This is relatively easy to do -- unless you want to hook into native functions, such as the accelerometer, camera, or even the Toast Messages (the little messages that pop up when something happens).

Accessing the native hardware and software through a webview requires you to write a JavascriptAdapter (available in Android 2+), and define some custom Javascript methods in the JavascriptAdapter. Those methods map to a java function, which DOES have access to the native OS facilities.

Frameworks such as Appcelerator and PhoneGap do exactly this, except they've already written the javascript functions for you, so it saves you from having to write those yourself.

In that sense, if you're going to write an app for Android using HTML / CSS / Javascript, it makes sense to use a framework. Otherwise, you're doing that work yourself.