Here are some of the features of the tool:
- It is pixel-perfect on the half pixel level for retina displays
- It has an optimized thin stroke width that is always one-sixth of the width of a period
- It sits on the optimal Y position between the baseline and descender line (the golden ratio point
- It has holes around descenders to completely respect the type’s shape
According to the developer, the concept of underline.js is in constant development and is designed to help push the boundary of web typography.
There’s more to learn about this tool as well if you are interested in the story behind its development. You can watch the video from the NY Tech Meetup where it was presented.