Prior to its launch on the Xbox One, Diablo 3 was rendering at 900p, that is, until Microsoft forced Blizzard to crank it up to 1080p. Blizzard announced during E3 a few months ago that the Xbox One version of Diablo 3 would be rendering at 900p, compared to the PS4 which was splashing things up onto TVs at 1080p.
A day-one patch for Diablo 3 on the Xbox One cranked the resolution up to 1080p, but saw performance issues. Not huge, but there are framerate drops during heavy gameplay in Diablo 3, something that didn't happen when it was rendering at 900p. Blizzard production director John Hight explained to Eurogamer: "We did find it challenging early on to get it to 1080p. That's why we made the decision to drop to 900. That's what we demoed and were showing around E3 time. And Microsoft was just like, 'This is unacceptable. You need to figure out a way to get a better resolution.' So we worked with them directly, they gave us a code update to let us get to full 1080p".