CSS positioned layout

The CSS positioned layout module defines how to position elements on a web page.

Reference

Understanding z-index: Presents the notion of stacking context and explains how z-ordering works, with several examples.
Stacking without the z-index property: The stacking rules that apply when z-index is not used.
Stacking floating elements: How floating elements are handled with stacking.
Using z-index: How to use z-index to change default stacking.
Stacking context: CSS stacking context, the CSS features that create new stacking contexts, and nested stacking contexts.

Specification
CSS Positioned Layout Module Level 3
CSS Positioned Layout Module Level 4

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https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_positioned_layout