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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 its artifact; under strict mode it stops.

What it looks like

target: book
fontsize: 13pt

WARN: fontsize '13pt' is not a size this set allows; using 12pt
  Valid: 10pt, 11pt, 12pt
  For a larger base size, switch to a KOMA target.
  See https://keystone.knight-owl.dev/errors/unsupported-fontsize/

A value that is no size at all reads a little differently, and falls back differently too:

WARN: fontsize 'large' is not a font size; using 11pt
  Valid: 10pt, 11pt, 12pt
  For a larger base size, switch to a KOMA target.
  See https://keystone.knight-owl.dev/errors/unsupported-fontsize/

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) takes 11pt instead. Either way the build continues with a real size, and says which one it used.

Take a size from the list the message prints

The allowed sizes per target are under Base font size. For a base size above 12pt, switch to the KOMA counterpart of your target.

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