Invalid attribute value

The attribute is one the shortcut takes, but its value isn't one the attribute accepts. The build warns, and the attribute's default stands in; it stops when the attribute is required and has nothing to stand in.

What it looks like

::: {.vspace size="banana"}
:::

::: {.aside type="todo"}
A callout whose type is not one of the defined kinds.
:::

::: {.multicol cols="9"}
Text that was meant to flow across columns.
:::

ERROR: shortcut 'vspace' rejects size 'banana'
  │ ::: {.vspace size="banana"}
  It takes a length in cm, em, in, mm or pt, or one of the values listed.
  The attribute is required, and nothing stands in for the value.
  Valid: huge, large, medium, none, small, tiny
  See https://keystone.knight-owl.dev/errors/invalid-value/
WARN: shortcut 'aside' rejects type 'todo'
  │ ::: {.aside type="todo"}
  │ A callout whose type is not one of the d…
  It takes one of the values listed.
  The default 'note' stands in.
  Valid: example, note, tip, warning
  See https://keystone.knight-owl.dev/errors/invalid-value/
WARN: shortcut 'multicol' rejects cols '9'
  │ ::: {.multicol cols="9"}
  │ Text that was meant to flow across colum…
  It takes an integer from 2 to 4.
  The default '2' stands in.
  See https://keystone.knight-owl.dev/errors/invalid-value/

What it means

A value you write is judged as written, an empty one included. One the attribute doesn't accept is dropped, and in its place stands:

• the attribute's default — multicol falls back to two columns, aside to a note — and the build warns;
• without a default, whatever the shortcut's own markup has there, else nothing, and the build warns;
• nothing, on an attribute the shortcut requires — vspace's size has no default — and the build stops.

The message names the shortcut and attribute as you wrote them, your own shortcut's included, and what the attribute takes. A strict build stops on the warning too.

Related

• An accepted value — a value your shortcuts.yaml writes, a default, a const or a template's, refused as the build compiles it.
• Undeclared mark — a set mark naming a mark you never declared.
• Unrecognized attribute — the attribute name itself isn't accepted.