This piece is spot on. For a glimpse into what such a design tool might look like, you could do a lot worse than Squarespace’s adaptive layout engine. It’s not meant to be a general purpose design tool, but rather, a special purpose tool meant to support the system it has in place. But within the use cases it’s meant to address, it does much of what Josh’s hypothetical design tool does. And best yet, it does it in live HTML. I would hope that if this design tool ever gets built, it takes a page from Squarespace’s book.