Readability has a LOT of hand tuned heuristics for figuring out the most likely content of the page, but the primary indicator on whether a tag with text in it is part of an article or not is the number of commas in the tag. It's my favorite thing about the algorithm because it's a dumb idea that works. The comma rule gets the extraction correct on about 70% of the web, the rest of the heuristics are mostly there to cover screwy ways people structure their articles.