I wonder if "word" is the correct unit, here; perhaps "morpheme" would work better, especially for multi-morphemic words or in compound nouns (e.g., in German).

I wonder if "word" is the correct unit, here; perhaps "morpheme" would work better, especially for multi-morphemic words or in compound nouns (e.g., in German). I think "syllable" may be a bit too much, unless the purpose was to teach reading. For example: ANTIDISESTAblishmentarianism (initial 40% highlighted) vs. ANtiDisESTAblishMEntARianIsm (highlighting rules over morphemes) vs. AnTiDisEstAbLIshMEntArIAnIsm (highlighting rules over syllables) An API that provided this as a service would be less trivial. The lay-typographer in me really hates the look of it too. Aesthetics don't trump readability, but perhaps there's a middle ground where the font weight drops off (e.g., with exponential decay), rather than having the binary "bold vs. not bold".