Looks like it gets data from cssstats.com:
https://api.cssstats.com/stats/?url=
Which is not very reliable/not able to parse some CSS files/rules (as one of the comments mentioned, !important is a problem)
EDIT: cssstat.com is pretty awesome: https://cssstats.com/stats?url=http%3A%2F%2Fmedium.com&name=...
Looks like it gets data from cssstats.com:
https://api.cssstats.com/stats/?url=
Which is not very reliable/not able to parse some CSS files/rules (as one of the comments mentioned, !important is a problem)
EDIT: cssstat.com is pretty awesome: https://cssstats.com/stats?url=http%3A%2F%2Fmedium.com&name=...