This sounds reasonable for web apps. Read the 69 comments for some great debate. I think the important take away is to design what is most crucial first and for web apps that's the interface (and it may be different for your particular application domain).

From Getting Real, Step 1: No Functional Spec (Signal vs. Noise).:

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Getting Real is all about starting from the user interface and customer experience and then building out. Visual design first, programming second. The more traditional process is starting from the abstract (documentation, diagrams, charts, etc.), coding a skeleton app, and then homing in on the real by finishing it up with an interface. We think that's backwards.

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