A reference to an Element representing the subject element whose visibility within its nearest ancestor scrollable element (scroller) will drive the progress of the timeline.
An enumerated value representing the scroll axis that will drive the progress of the timeline. Possible values are:
"block": The scrollbar on the block axis of the scroll container, which is the axis in the direction perpendicular to the flow of text within a line. For horizontal writing modes, such as standard English, this is the same as "y", while for vertical writing modes, it is the same as "x".
"inline": The scrollbar on the inline axis of the scroll container, which is the axis in the direction parallel to the flow of text in a line. For horizontal writing modes, this is the same as "x", while for vertical writing modes, this is the same as "y".
"y": The scrollbar on the vertical axis of the scroll container.
"x": The scrollbar on the horizontal axis of the scroll container.
A value or array of values representing an adjustment to the position of the scrollport (see Scroll container for more details) in which the subject is deemed to be visible. Possible values are:
"auto": The default box position is used.
A string: If a string is specified, it can consist of one or two values equal to auto or a CSS <length-percentage> value. To put it another way, the string should be a valid view-timeline-inset value.
An array of one or two values, which can be "auto" or a suitable CSSNumericValue to represent a length or percentage offset (for example CSS.px() or CSS.percent(). If an array is provided, the first value represents the start inset (which affects the ViewTimeline.endOffset value) and the second value represents the end inset (which affects the ViewTimeline.startOffset value).
If the array has only one value, it is duplicated.
