Implement CryptoAlgorithmRSA_PSS for OpenSSL. In the CMake files add a check for the necessary API. The API landed in OpenSSL 1.1.1 but LibreSSL always reports as 2.0.0 in CMake so it might require more work.
I'm addressing this in Bug 225294. RSA_PSS support can be detected with the EVP_PKEY_CTX_set_rsa_pss_saltlen macro. In the case where it's not defined, it should be okay to simply return a NotSupportedError exception from the view point of WebCrypto API.
CryptoAlgorithmRSA_PSS for OpenSSL has already been implemented in Bug 225294. Let me change the status to RESOLVED as DUPLICATE. *** This bug has been marked as a duplicate of bug 225294 ***