OptionalcontentPer-item override of the feed-level fit.
OptionalisMarks an ad item: "Sponsored" chip, never loops, auto-advances.
OptionalposterPoster shown for pages outside the player window. Falls back to
media.posterUrl.
OptionalrailUp to six host buttons for the right rail.
OptionalsecondarySecond source for the split-video layout: plays in the bottom half, starts in the same frame as the primary, pauses/resumes with it.
OptionalsplitWhich half of a split-video page is audible (default PRIMARY).
OptionalsubtitleOptionaltextOVERLAY (default) or one of the split text layouts.
Optionaltitle
One entry of a vertical feed: media plus feed-specific presentation.