Tempo text is bold plain type; expression text is italic — the two must not be visually or semantically conflated

A tempo marking (Andante, ♩=96) is set in bold, non-italic (roman) type above the top stave; an expression marking (dolce, espress., cantabile) is set in italic type, conventionally above the staff near the note it modifies. The two serve different functions and must be visually distinguishable.

Gould, Behind Bars, ch. Tempo Indications and ch. Expression Marks — gives the bold-roman vs italic distinction explicitly; Ross, The Art of Music Engraving, agrees.

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