After spending some time looking for a simple lightbox solution, I was left surprised that almost all of them use jQuery, which I didn't want to use in my projects anymore (unless necessary).
Thanks to new EcmaScript standards, it is now easier than ever to build such things without any dependencies, so I decided to give it a go. I started to develop a custom lightbox without any third-party scripts with the same functionality as existing solutions but below the limit of 15 KB.
Litebox is the result of that work. It's an image-only lightbox with customization options (selectors, labels, controls, animations, etc.) and weighs around 12 KB. Currently, at version 0.8.3, it's still in development but can be used on websites today.
The development has come to hold in the last two years, but I plan to continue working on it eventually.