The other day I did a post on how to create a self-signed certificate using PKI.js in that sample we included a Basic Constraints extension but we could have also just as easily defined a custom or new certificate extension. For example thanks to #heartbleed folks are talking about MUST STAPLE again, this is an extension that was proposed several years ago that when present would indicate that clients should hard-fail instead of soft-fail with OCSP.

This proposal is based on a generic concept of expressing a security policy within the certificate. While the OIDs for this extension and the associated policy have not been defined yet one can easily construct a certificate using this extension with PKI.js:

cert_simpl.extensions.push(new org.pkijs.simpl.EXTENSION({
    extnID: "1.2.3", // No OIDs assigned yet
    critical: false,
    extnValue: (new org.pkijs.asn1.SEQUENCE({
        value: [
                   new org.pkijs.asn1.INTEGER({ value: 4 }),
                   new org.pkijs.asn1.INTEGER({ value: 5 }),
                   new org.pkijs.asn1.INTEGER({ value: 6 })
               ]
               })).toBER(false)
}));

NOTE: In the above snip-it we just made up two OID values, hopefully IANA will assign OIDs soon so it is possible for browsers and CAs to implement this extension formally.