We strongly believe that the only way to know what something we really look like is to work in the medium in which it will be ultimately be presented in.
Anything less will just be an approximation of the final product.
Well, I can't really agree with this. Filming a movie and viewing a movie are two completely different experiences and take place in completely different environments, even if the medium is the same.
What you're saying would be like saying all movies should be filmed in cinemas… I think using the browser's rendering engine is great, but there's no reason why we necessarily need to create websites in actual browsers.
In fact I think the main weakness of apps like Easel compared to, say, Photoshop, is precisely the fact that they run in the browser and don't feel as fast and responsive as a native desktop app.
The analogy doesnt really hold up considering how much time/money have been invested in making realistic previews available on cameras, that is the entire point of an slr.
Being inside a browser does not equate to slow, and using photoshop as an example pretty much proves that point.
Anything less will just be an approximation of the final product.