Hi,
interesting question indeed. I would not bet on this tho, at least for the first few iterations. Flash failed because of unsupported api features that was critical for many assets and projects, including PlayMaker, so if for example, .net reflection or some collections are not supported, webgl support within Unity will be very loose and far from a plug and play publish... The same happened with windows publishing during the first few iterations, it was a very messy situation because Unity was not fully ported there for various reasons.
bye,
Jean