At one point we thought constructor= could point to an existing function, and hence its prototype would be used (and checked that the base element’s prototype was on the prototype chain.)

It would be nice to design something that was compatible with user-defined constructor bodies in some future version.

The current situation is tenable; authors could just mix in behavior to the prototype object that is created by the constructor generation algorithm.

If authors can specify a prototype, there’s a semantic question – do they specify the object which is first link in the prototype chain, and a separate prototype object is still synthesized; or do they specify the generated constructor’s prototype directly?

The former case (prototype object is always synthesized) leaves open the possibility of requiring GeneratedConstructor.prototype.constructor === GeneratedConstructor and implies that there is at least one unique prototype object per custom element. These might be useful properties for implementers to exploit.

Defined in http://dvcs.w3.org/hg/webcomponents/rev/e6edb53aa5d9 and on.