Can I Build an IM App or a Wiki from Scratch Using Only HTML and CSS?

You could technically create a wiki with just HTML and CSS, but it would mean writing every single page and link manually. For anything beyond simple linking and categorization, like search functionalities, you'd definitely need a database and a backend language. An instant messaging client like Discord? That one absolutely requires a database, a backend, and some type of front-end socket functionality to work smoothly. And yes, you'd need some server to host it, whether it’s a shared one or dedicated really depends on your budget and expected traffic.

Just a heads up, before diving into this project, check out MediaWiki. It’s a GPL-licensed software developed by the WikiMedia Foundation that could really kickstart your wiki ambitions!

For a basic IM client, you actually don’t need a database if you're okay with the messages not being stored long-term. You could use local storage for that. And while you need some backend for establishing connections, once that's done, you can use WebRTC to handle peer-to-peer connections without a complex backend for messages.