It's using this code: https://github.com/mozilla/readability/blob/master/Readability.js https://github.com/mozilla/readability/blob/master/Readabili... which

It's using this code: https://github.com/mozilla/readability/blob/master/Readability.js https://github.com/mozilla/readability/blob/master/Readabili... which is a descendant of the original Readability.js published by Arc90, before they decided to turn it into a proprietary Instapaper clone. It extracts what looks like article text from the page markup. DOM elements included for a printer-friendly view could possibly be helping, but it doesn't target that directly.