A spec does not need to be heavy to be useful. The point is to name the audience, the job of the page, the content model, and the failure states before the layout starts accumulating clever exceptions.
For a small static site, I like to write the routes first, then sketch the repeated components, then list the exact checks that should pass before publishing. That gives the design a useful amount of friction.
The payoff is calmer implementation. Every card, link, preview, and generated page has a reason to exist, and visual polish can happen without losing the structure underneath it.