> what is the analogue of syntax highlighting for natural languages
Nouns begin with a capital letter in German, and used to be in Dutch and English until a few hundred years ago. Perhaps it was because nouns are generally spoken with the greatest stress of all words in an English sentence. Perhaps such capitalization is an analogue of programming syntax highlighting.
Sometimes long complex English sentences can be very confusing to me, especially in finding the verb in there. When two or three consecutive words have different grammatical meaning, and can be used in different grammatical meanings, it can become very confusing if English is not your mother tongue. In that case, color coding (or bold / italic / underscore) could help a lot. But for a word processor to do that, it has to understand grammar...
I've tried part-of-speech tagging and subjectively it doesn't seem to help much. POS taggers for English are a solved problem, so it can be done automatically, but for prototyping I tried marking up some text myself. Indeed what helped the most were the verbs (vs other parts of speech), but it still didn't seem to be worth doing.
Also marginally helpful was to delimit noun / verb phrases with 2 spaces instead of one.
I think of it as a space to explore. Punctuation is another preexisting visual aid in parsing. However, printed text is rather limited compared to the ways a computer can style text dynamically for you.
Edit: The fact that this capitalization disappeared suggest the returns from the effort of capitalization weren't so great. But now the investment per word is much lower because the process can be automated.
Nouns begin with a capital letter in German, and used to be in Dutch and English until a few hundred years ago. Perhaps it was because nouns are generally spoken with the greatest stress of all words in an English sentence. Perhaps such capitalization is an analogue of programming syntax highlighting.