position: absolute; not work

position: absolute; not work

https://stackoverflow.com/questions/11928294/css-position-absolute-with-position-relative-top-not-working

https://stackoverflow.com/questions/3830486/absolute-position-is-not-working

CSS Collapsing Margins

https://css-tricks.com/what-you-should-know-about-collapsing-margins/

https://stackoverflow.com/questions/43514507/why-top0-with-positionabsolute-not-working-if-another-element-has-margin-top-h

css margin 合并

css 塌陷

https://codepen.io/xgqfrms/pen/PoqLBZN

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