It seems like we expose length attributes for all of the *KeyAlgorithm interfaces besides HmacKeyAlgorithm.  Also, the other *KeyAlgorithm interfaces have the [NoInterfaceObject] directive.

Proposed micro-patch:
+[NoInterfaceObject]
 interface HmacKeyAlgorithm : KeyAlgorithm {
   // The inner hash function to use.
   readonly attribute KeyAlgorithm hash;
+  // The length (in bits) of the key
+  readonly attribute unsigned long length;
 };