https://www.youtube.com/watch?v=7SRTEXSpcyI
right now, and looking at the Titanfall video, it seems like 60 fps is only available if you're loading the DASH streams (fmt 136, 137, 247, 248)
136 mp4 1280x720
137 mp4 1920x1080
247 webm 720p
248 webm 1080p
on
https://www.youtube.com/account_playback , selecting "I have a slow connection. Never play higher-quality video" will default to fmt 18 (mp4 360p) or 243 (webm 360p)
I don't know if they will change 480p, 360p, 240p, or non-DASH formats to allow 60 fps
YouTube's DASH has separate audio and video streams, and multiple segments of video that must be requested individually, YouTube's player will automatically switch resolutions in the middle of playback depending on how far the grey bar gets ahead of the red bar, or if the red bar completely gobbles up the grey bar.