Yes you can if you embed everything in a webview, but it's a waste. You won't take advantage of the real power of Titanium - native apps. It will be nothing more than a PhoneGap replacement. Don't get me wrong, PhoneGap is great, but is not suited to big complicated apps and the speed is way under what Titanium can deliver.
You can't build Titanium app using HTML and CSS. Titanium provides JS environment which is different from browser DOM environment and therefore HTML and CSS are not applicable in it (they can only be used in WebView, which provides DOM environment).
You must use JS and JS API that Titanium provides. For examples check Wiki and official documentation: http://developer.appcelerator.com/apidoc/mobile/latest
If that's really what you want to do, don't use Titanium. There are other tools out there that do exactly what you want.
Think you can help? Login to answer this question!