Interface started out in late 2016 as an experiment to build a perfectly pixel–fitting font at a specific small size (11px.) The idea was that by crafting a font in a particular way, with a particular coordinate system (Units Per EM), and for a particular target rasterization size (11), it would be possible to get the best of both sharpness and readability.
However after a few months of using an early version of Interface, it dawned on everyone exposed to the test that this approach had some serious real–world problems. Most notably that it was really hard to read longer text. Because of the pixel–aligning nature of that approach, the font took an almost mono–spaced appearance, making it really easy to read numbers, punctuation and very short words, but eye–straining to read anything longer.
The project was rebooted with a different approach, sticking with the specific UPM, but crafting glyphs and kerning in a way that made for more variation in the rhythm and smoother vertical and horizontal stems. As Interface was being developed, it was tested on an internal version of Figma—where the author of Interface works as a designer—and slowly improved upon based on experience and feedback.