I love projects where people make things just because they can rather than because a client wants them to and 98.css is definitely one of these.
The project allows you to create Windows 98 inspired UIs, including buttons, windows and treeviews.
All you have to do is drop a CSS file on your page and you are good to go.
<link rel="stylesheet" href="https://unpkg.com/98.css">
Here are some example components:
Check 98.css out and have a look at open issues to see if you can contribute to this excellent project. I have even made a tiny contribution to the project myself.