Unsupported font size

fontsize names a size the active target's document class doesn't offer. Keystone substitutes the closest allowed size and warns, so a lenient build still produces a book; under strict mode it stops.

What it looks like

target: book
fontsize: 13pt

WARN: fontsize '13pt' is not supported (allowed: 10pt, 11pt, 12pt); using 12pt. For a larger base size, switch to a KOMA target — see https://keystone.knight-owl.dev/configuration/targets/#base-font-size

What it means

Each target allows a fixed set of base sizes — a property of its document class. A size outside that set snaps to the nearest allowed one; a value with no recognizable size (a wrong unit, non-numeric text) resets to the default instead. Either way the build continues with a real size rather than the silent fallback LaTeX would otherwise apply.

How to fix

Pick a size the target allows, or — for a base size above 12pt — switch to the KOMA counterpart of your target. The allowed sizes per target are listed under Base font size.

Related

• Invalid attribute value — the same warn-and-fall-back shape for a handler attribute rather than a metadata value.