Vanishing point
The tiles tip back near the pointer. They sit at the top of a long column with a perspective, and the red cross marks the column's perspective-origin: the point the 3D is seen from. With perspectiveOrigin on, the effect puts it at the centre of the tiles. Turn it off to see the browser's default, the middle of the column, far down the page, so the tiles are seen steeply from below.
A perspective on an element gives its children's 3D transforms depth, seen from a single point: the perspective-origin, which is the middle of the element unless it's set. On a small stage that wraps the animated elements closely, the middle is where you'd want it.
When the element with the perspective is much bigger than the animated ones, such as a long column like this one, or the page itself, its middle can be a long way from them. Elements near the top are then seen from far below, and stretch and lean towards a point you can't see. Elements further down the page look different again.
Setting perspectiveOrigin: true moves the origin to the centre of the animated elements, measured as they are at rest, and keeps it there when the page reflows. The elements need to share a parent, since that's the element whose origin is set; if they don't, the effect logs a warning and leaves the origins alone.
Another option is perspective, which gives each element a perspective of its own, seen from its own centre, in place of the parent's. Then every element looks the same wherever it is, rather than all of them sharing one vanishing point.
The rest of this column is only here to make it long.
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