My first version of Intersection Observer will disallow cross-document observing, for simplicity. This bug tracks supporting cross-document observation.
It's a tragedy this wont land in iOS 11 or Safari 11, those being non-evergreen browsers. My first request for this functionality was in 2014 and Safari is the only major browser to not implement a mechanism for cross document in view measurement. There are mechanisms for measurement all the way back to IE9 and tons of destructive/invasive hacks like frame busting and geometric guessing that are being used today, including in some cases measurement using plugins.
It seems this is somehow a lower priority item than other enhancement requests, but Safari is most mobile web traffic and still relevant in desktop browser markets as well. So long as those browsers don't report in view dims, fraudsters will target iOS applications and safari web browsers, and safe measurement techniques will not be available, making energy/computationally expensive and potentially unsafe operations for measurement the only way to combat bad actors on the web.
For more information about "Viewability" measurement guidelines, see the following pages provided by standards leaders in advertising, IAB and MRC found here:
Mobile App: http://mediaratingcouncil.org/Mobile%20In-App%20Measurement%20Guidelines%20(MMTF%20Public%20Comment%20Draft%20Final).pdf
Mobile Web: http://mediaratingcouncil.org/Mobile%20Web%20Measurement%20Guidelines%20(MMTF%20Public%20Comment%20Draft%20Final).pdf
Mobile (Guidance): http://mediaratingcouncil.org/062816%20Mobile%20Viewable%20Guidelines%20Final.pdf
WICG developed standard and examples: https://github.com/WICG/IntersectionObserver
And here's where we stand today regarding universal availability on browsers: http://caniuse.com/#feat=intersectionobserver
A deep dive (video) into ad fraud can be found here: https://www.youtube.com/watch?v=tgtH1L57iLc
All this is to say, we very much are ready for this feature out in the wild. It's sorely needed on Safari browsers, and it is a high value feature for the mobile web and mobile app ecosystems and this helps reduce payouts for bad actors.