jQuery214013829352706670761: 3

is added to the document object, but I can't figure it out.  That propname matches the "expando" prop for a given jQuery load, which suggests that this has something to do with events, but I can't figure out where, when, or what the 3 means.  I've searched uses of expando and also document in the jQuery source, but nothing looks quite on point.

Why is this important?  Because I've discovered, using Object.observe(document) that it's happening at a very particular, and somewhat fascinating, time:  http://jsfiddle.net/LpqLswu2/10/  (check the very first console log: it's catching jQuery adding that prop to the document object via Object.observe)

Namely, the prop isn't added until document.body legitimately exists, but it's added at least several milliseconds BEFORE DOMContentLoaded is fired (and the gap widens the more dependencies are added to the head), meaning that if I can figure out what method is being used to add that prop, I might have a way of loading iframes slightly faster than previously known methods.