The latest version of BrowserPrint (v18.104.22.1689) doesn't seem to work correctly in Safari (12.0.3).
When trying for example the Zebra Sample that's included when installing the BrowserPrint application you get the following error messages when trying it in Safari (works fine in Chrome/FireFox):
You can try the sample here:
I suspect it's because it triggers a HTTP request instead of a HTTPS request because the zebra file is served over HTTPS and because of that Safari blocks this request. It would work if the resource would trigger a request to https://127.0.0.1:9101 but then you get the certificate issues again...