YouTube playback is seamless on most devices, but there are some known issues with 'automatic playback'.
iOS - iPhone, iPad
Automatic playback is not possible on iOS. Instead, students simply hit 'Play' on the YouTube video. Any tonic or count-in will play first, and the YouTube video immediately after.
Chrome - Embedded LMS Launch
When doing an 'embedded' launch from your LMS, automatic playback is not supported.
Playback of the tonic and count-in will happen automatically, and students then simply hit 'Play' on the YouTube video. Subsequent plays of the same question using the replay button will play any tonic and countin, and video, with a single press of the replay button.
To workaround this issue, simply have your launch open in a 'New Window' or 'New Tab'.