CSS z-index

The arrangement of elements on a webpage is typically controlled by the CSS attribute "z-index". In this context, "stacking" pertains to the visual organization and layering of elements above one another within a website. It determines the priority of elements, dictating which one appears foremost and which one remains in the background when multiple elements occupy the same space on a webpage.

The Z-axis, also referred to as the depth or "third dimension" of a webpage, is managed by the Z-index property. This property determines the stacking order and presentation of elements on the webpage.

Example:

.container {
  position: relative;
}
.box {
  position: absolute;
  z-index: 2;
}

In the above example, the .box element will be layered above other elements within the .container element due to its higher z-index value.

Advanced Concepts of Z Index

• ✅ Stacking Contexts: When an element is positioned with a z-index value, it creates a stacking context. Each stacking context is an independent layer, and elements within a stacking context are only compared to each other when determining their stacking order.
• ✅ Stacking Order: Stacking orders determine how elements are layered within a stacking context. Three factors determine the stacking order: The stacking context with a higher z-index value is placed on top of a stacking context with a lower value. The element that occurs later in the HTML structure is shown on top if two elements have the same stacking context. If two elements have the same 'z-index' value and a similar stacking context, their stacking order is determined by the order in which they appear in the HTML structure.
• ✅ Negative z-index: The default stacking order or items with a negative "z-index" value are placed below those with a positive value.
• ✅ Positioning Contexts: Elements with different positioning types (position: static, relative, absolute, or fixed) create their own stacking contexts. For example, an element with position: relative and a z-index value will be layered within its stacking context, independent of other stacking contexts.
• ✅ Sticky Positioning: Elements with position: sticky are positioned according to their normal position in the document flow until a specified threshold is reached. Sticky elements are treated as if they were position: relative until the threshold is met, and then they become position: fixed. Elements with position: sticky can have a z-index value to control their stacking order within their stacking context.
• ✅ Parent-Child Relationship: The stacking context of a parent element determines the 'z-index' value of a child element. The stacking order of the child element inside that context may change if the parent element has a greater "z-index" value.
• ✅ Transparency and Stacking Order: Elements with transparent backgrounds or opacity values less than 1 can affect the stacking order of overlapping elements. The transparent or semi-transparent areas are treated as empty, allowing elements behind them to be visible and potentially affecting the stacking order.
• ✅ The stacking context with a higher z-index value is placed on top of a stacking context with a lower value.
• ✅ The element that occurs later in the HTML structure is shown on top if two elements have the same stacking context.
• ✅ If two elements have the same 'z-index' value and a similar stacking context, their stacking order is determined by the order in which they appear in the HTML structure.