I don’t think we should be using static ‘pixel-perfect’ mockups as the gold standard to what a interface build with code should look like.
Thats like using an illustration instead blueprints to engineer a house.
Static mockups should be used as a guide to inform the interface development, but not be replicated exactly. Design is a collaboration between designers and engineers; engineers shouldn’t be trying to perfectly replicate a mockup.
From a technology perspective, the interesting thing about the whole wysiwyg thing is that an interface is tied to the ‘complex logic’ that drives it. So if you were to use a tool that created your coded interface you would end up having to retrofit your technology to the manufactured interface, which could present limitations on your end product.
For standard things like static websites wysiwyg is great, but we already have tools for that.