AES CFB input parameter "s"

All,

Apologies in advance if this has been discussed before, or has a well-known
answer.

According to NIST SP800-38A [1] Section 6.3 which we reference for AES CFB,
the input parameters to the encrypt/decrypt operations consist of an IV,
the plaintext / ciphertext (respectively) and an additional parameter, s,
which takes a value between 1 and 128, inclusive.

What values of this shall we support in WebCrypto ?

I assume that if we wish to support more than one value, then we must add a
property to AesCfbParams for this.

...Mark

[1] http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf