I've been wondering what is the analogue of syntax highlighting for natural languages. Still experimenting with it, but most promising is simple keyword highlighting. I've written a Chrome extension that does this:
https://chrome.google.com/webstore/detail/highlit/cooahmcpma...
Interestingly, keyword highlighting apparently helps some type of dyslexic people focus their attention on the text. With the web full of distractions, I've noticed similar effect myself. Keywords also seem to help skimming, when you're reading the text with a specific purpose in mind.