Folks,
As you are all well aware, we have had extensive discussions in this WG (on both the list and during our conference calls) on the need for defined extensibility points in the WebCrypto specification. The result of those discussions was an agreement that those defined extension points would be added to the specification as part of resolving Bug #25618 (https://www.w3.org/Bugs/Public/show_bug.cgi?id=25618), which was the placeholder for this work. Co-editor Mark Watson has already made those changes to the draft and asked for them to be reviewed.
Speaking on behalf of Microsoft and our two independent implementations of WebCrypto (Internet Explorer 11+ and the MSR JavaScript Cryptography Library), we believe that the spec should not exit Last Call without having a well-defined extensibility mechanism that allows the definition and integration of new cryptographic algorithms. Our expectation is that whatever the mechanism, an extension will not impact the base specification nor compromise implementations that comply with the base spec.
--bal