I've just poked through both the GetPocket site (https://getpocket.com/publisher/) and Mozilla's Readability Library GitHub page (https://github.com/mozilla/readability) without seeing obvious guidelines.

My general suspicion is that adhering to a simple HTML5 documemnt structure, and possible use of microformats (https://microformats.io/) goes a long way.

Update: there's some discussion here: https://news.ycombinator.com/item?id=28301113