A couple lines of CSS to improve the user experience of Special:ListGroupRights
I add line numbers to Lua modules with JS & CSS.
An improvement to the Cargo pagevalues view using just JS & CSS.
Here's how I've built a framework for both static and lazily-loaded popups.
Disambiguations are typically somewhat user-unfriendly; I describe my method for improving quality of life when disambiguation links do appear.
The Gadgets extension is a great way to organize code instead of using Common.css and Common.js for everything.