How to load a PKCS#12 Digital Certificate with Javascript WebCrypto API

I'm trying to sign data using the WebCrypto API, but instead of creating a private/public key and exporting it to pkcs#1 or 8, I would really like to use a user's PKCS#12 to sign data. I've read the W3C spec, but cannot make much of it and can't find any good material on how to do this. Right now I want to leave ActiveX and Java Applets aside. Is there a way to tweak the following:

var buffer = encode(prompt("Please enter your password"));
    //TODO:
    //implement a prompt for a pfx or cert

  return crypto.subtle.importKey("raw", buffer, "PBKDF2", false, usages);
    //TODO:
    //instead of importing it, ask for the certificate's pass to sign data
    //with crypto.subtle.sign

Any pointers?

UPDATE Here's the code I've been working

<script src="forge.min.js"></script>

<script>
    var errorsReportedByVerifier;
    errorsReportedByVerifier = checkStorage() && checkBrowserAPIs();
    if (!errorsReportedByVerifier){
        console.log("adding click event");
        document.getElementById('btnPfx').addEventListener('click', handlePFXFile, false);
        storeVariables();
        getVariables();
    }


    function handlePFXFile(evnt) {
        console.log("handling pfx")
        //alert(document.getElementById('pfx').value);

        //error happens in 1st line
        //error object does not accept property replace
        //forge.min.js Line 1, Column: 17823
        var p12Der = forge.util.decode64(document.getElementById('pfx').valueOf());
        //var pkcs12Asn1 = forge.asn1.fromDer(p12Der);
        //var pkcs12 = forge.pkcs12.pkcs12FromAsn1(pkcs12Asn1, false, 'pss');
        console.log("pkcs12");
    }
</script>