Material: z-index - CSS: Cascading Style Sheets

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z-index - CSS: Cascading Style Sheets | MDN

The z-index CSS property sets the z-order of a positioned element and its descendants or flex items. Overlapping elements with a larger z-index cover those with a smaller one.

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CSS rule: z-index

External link: https://developer.mozilla.org/en-US/docs/Web/CSS/z-index