CSS rule z-index - the position of the page element in the Z coordinate: the display level of the element or the layer in which it is located. A tag with a larger z-index will be displayed in full. Tags are displayed in the order they appear in the input stream and overlap each other. The z-index value determines the visibility priority.
We must pay tribute to modern browsers and algorithms for displaying elements. Since the time when the graphics took possession of the screens and the problem of clipping visible and invisible parts of elements in application windows was formed, the technology for displaying visible content has achieved excellent results. In the browser window, all the elements are displayed correctly, the user sees only what is defined by the design or developer.
General rule: order and level
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However, if you transfer the semantics of z-index to the area of dialogue with the visitor, you can create a practical effect. Similar to how tags can overlap, you can impose dialogs (site-visitor) and move between them. From this point of view, applying the CSS z-index rule seems very practical and practical.