Please support an optional hint inside the cloze syntax. While the text is hidden, the hint shows in the blank; once revealed, only the answer remains.

Photosynthesis needs {{1::light, water and carbon dioxide::3 items}}.
  hidden:   Photosynthesis needs [3 items].
  typing:   the input box shows "3 items" as greyed placeholder text,
            which disappears as soon as you start typing.
  revealed: Photosynthesis needs light, water and carbon dioxide.

The Treaty of Tordesillas was signed in {{1::1494::year}}.
  hidden:   The Treaty of Tordesillas was signed in [year].

(The brackets only mark the blank; the look is up to you.) When typing, the hint is never part of the expected answer: "light, water and carbon dioxide" is checked, "3 items" is not.

Why: some cards keep the exact wording of a source (a textbook, a law, a standard), so the sentence can't be rewritten around the blank. Then some blanks give no orientation on their own: a list hidden in one blank, a number, a date. Today I write the hint as text after the blank, {{light, water and carbon dioxide}} (*3 items*), but that is only a workaround: it stays visible after the reveal, in grouped cards it also shows in the variants where its blank isn't being asked, and when typing it sits outside the input box.

Anki has something similar ({{c1::answer::hint}}), but there the hint only fills a static blank. In Mochi it should also work while typing, as the input's placeholder.

Compatibility: hidden text without a hint doesn't change. One detail to settle: a bare number before :: already means a group, so {{1494::year}} would be read as group 1494. The hint could be allowed only in the grouped form ({{1::1494::year}}), or get a separator of its own. Your call.

It also pairs with the fixed-width request: the blank would show the hint when there is one, and the neutral mask otherwise.