A rendering of every block with the current theme styles applied, in one scrollable view, which is a design review artefact nobody had to build.
what it caught, on the first look:
the quote block, unstyled since the theme was written
a table with no border colour, invisible on the dark
variation
three heading sizes that did not follow the scale
a button with a focus ring that matched the background
four issues, ten minutes, on a theme that had been
reviewed page by page for a year.
Reviewing by page finds what is on the pages, and a block nobody has used yet is unstyled until the day somebody uses it. The Style Book renders the whole vocabulary regardless, which is exactly the artefact a design system needs and which we had been maintaining by hand in a separate page template. That template was deleted.