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https://drafts.css-houdini.org/css-layout-api/#interaction-positioning
Should the example in the Positioning section have a #child-absolute { position: absolute; } rule in the style sheet? It would be good to also explain what the resulting position of this element is (i.e. (0, 0) due to the rule for computing static positions above).
Is the comment /* container is a containing block */ necessary? This makes it sound like it is only the position: relative; declaration that causes the element to be a containing block, but because it is display: layout(foo); it'll be a containing block anyway. It might be clearer to remove that declaration and explain why it's a containing block anyway.