It's js only, nothing is sent to the server. It automatically makes the background of signatures transparent. The result is a raster pdf as if you printed, signed and scanned the document.
I use it on desktop, not sure if it works well on phones.
Let’s Encrypt cross-signature with IdenTrust "DST Root X3" is ending on September 1, 2021 but 33.8% of Android devices are running versions under 7.1 which don't trust Let’s Encrypt new root certificate "ISRG Root X1"
[1] https://community.frame.work/t/noisy-psu-fan/74751