Android 8.0 Oreo, thoroughly reviewed
If it's true that Chrome on Oreo genuinely prevents picture-in-picture for YouTube specifically, that's a troubling precedent to set. Why not allow all websites to enable/disable PiP (maybe via a meta tag, in the way that tab theme colours already work)? If I query https://m.youtube.com while emulating a Nexus 5X, the response contains:  

Is that what is preventing PiP on YouTube or is it some sort of underhanded inter-Google arrangement between Chrome and YouTube?