It is DER and it follows the RFC for TLS v1.2. Opened up WireShark and exported the raw bytes for one of the certificates among the chain transmitted by the server in the SSL / TLS “Certificate” message and decoded it and validated the DER encoding. This was on an HTTPS connection to an Apache Web Server.
The other question that web server administrators and writers of server certificate verification code would need to know is what should be the order of the certificates in the certificate chain that is being sent back by the web server. The RFC provides details on that as well wherein the sender’s certificate must come in first followed by the certificate that would certify it and so on.