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Just a little correction: percentage values are computed based on the element's _containing block_, not parent.

Most of the time, an element's containing block is its parent, but not always.

For more on this: https://developer.mozilla.org/en-US/docs/Web/CSS/Containing_...

I found the rule about absolutely positioned elements interesting given that I often thought their containing block was the closest relatively positioned parent. As outlined in that documentation, though, it's just the closest element that is not statically positioned.

Oh, wow. That explains a few baffling bugs I’ve seen.

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