A few things to note:
Character rendering bugs
I have seen glitches that result in rendering bad Cistercian characters in a couple of browsers, namely Firefox on macOS and the browser built-in to the iOS Slack client. I'm not sure what causes this - maybe ligature bugs in some browsers? If you encounter a bug, please take screenshots and describe what you're doing and open a bug report, and if you know what might be causing these, I'd appreciate any help.
Copy and paste will not work
Cistercian characters are not (yet?) part of Unicode, and therefore the FRBCistercian font places them in the Private Use Area of Unicode. This means that, while you can copy the Cistercian character generated here like regular text, pasting the result won't make sense anywhere else that doesn't use the FRBCistercian font.
Historically, Cistercians did not record zero and technically there is no representation of zero in the original system. FRBCistercian uses the plain stave to represent zero.
Nonstandard 5 characters
The representation of 5 in FRBCistercian, of a triangle, is nonstandard; a more standard notation would be a dot.
Cistercian was written either vertically or horizontally. FRBCistercian uses a vertical stave.
Development and source code
You can find the source for this project on GitHub.