Alto clef is the viola's primary clef and is correct wherever the viola's normal register sits

Alto clef (C clef on the middle line, so middle C sits on the staff's third line) is the viola's standard clef because the instrument's core comfortable register (roughly C3 to C5, tessitura table below) straddles the bass/treble boundary in a way neither clef serves well — alto clef puts the bulk of that range on the staff with minimal ledger lines in either direction. Outside the viola, alto clef appears in Gradus's curriculum context wherever open-score fugue writing puts an inner voice (alto-range vocal part, or an instrumental line in that tessitura) on its own stave, and in trombone parts written unusually high (rare, and usually tenor clef is chosen first — see clef-choice-tenor-clef).

A viola part notated in treble clef throughout because the JSX renderer does not support alto clef (see gradus-jsx-renderer-treble-bass-only-cost below) — every note in the viola's comfortable range then either sits low in an unnaturally ledgered treble stave or is silently octave-transposed to avoid it, both of which misrepresent the instrument.

Gould, Behind Bars, Clefs chapter, states alto clef as viola's standard clef without qualification. Adler, The Study of Orchestration (the standard orchestration-pedagogy reference, cited here as the authority for the instrumental-tessitura claim specifically since it is not one of the four notation authorities named in this rulebook's brief but is the standard cross-reference for 'why alto clef exists') gives the viola's core range as squarely alto-clef territory. Ross and Read do not add anything beyond confirming alto clef's C3 stem-direction/middle-line construction (already documented in the Stems and Flags chapter's `MIDDLE_LINE_DIATONIC.alto = 28`, i.e. C4).

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.