Web development without using HTML/CSS/JavaScript

You can use Python with Pyjamas, Objective-J (and a little JavaScript) with Cappuccino, or Java with GWT.

If you don't mind being reliant on browser plugins, you can also look at Adobe Air, Adobe Flex, Flash, Silverlight, or JavaFX (or just plain Java).

To answer your other question, most web apps are written in HTML/CSS/JS because those are what most browsers can run without help. Everything I mentioned above either compiles to those or relies on the user having a plugin installed.

Yes, it is possible you could use Silverlight or JavaFX (even though it would require a minimum of HTML like 4 tags...).

The problem with Silverlight or JavaFX and Flash at the same time is that they suck at SEO, that is search engine optimization, as Google & co can't look inside your app.