Generating Safari developer certificate from Windows? The old instructions that have since been removed
This how-to might contain all necessary information provided by Apple in the past or help otherwise:

The Apple Safari extension development guide describes how to generate CSR(Certificate Signing Request) file for Mac. But, if we want to develop the Safari extensions on windows 7, we don’t have any resource to guide us. Here, I am providing steps to create  developer certificate for windows.


Register on Apple Safari developer program here
Download and install X Certificate and key management (XCA)
Generate a new private key in XCA. - After installing the XCA, go to File ->New Database. Give database name and password. Now, the new key option will be enabled.
Choose new key and give the name for private key and give size as 2048 Bytes.
From the XCA, choose “certificate signing requests” tab. Click on “New Request”  and fill the necessary details.
Export the Certificate Signing Request(CSR).
Request a certificate at apple here using the Certificate Signing Request(CSR).
Download the certificate from Apple account.
Click on “Certificates” tab on XCA and import the certificate which is downloaded from Apple into  XCA.
Export the certificate as p12 file.
Double click on *.p12 file. The certificate will import into the windows Certiciate-Manager.
Now, open the safari browser and click on Develop -> Show Extension builder. You are able to see that the certificate got detected.