Preparation
🎨 ⏐ CSS3 Questions
9. z-index

z-index

Understanding the z-index property in CSS is crucial for controlling the stacking order of elements on a web page. Let's dive into its usage:

Definition:

The z-index property controls the vertical stacking order of positioned elements that overlap. It specifies the stack level of an element along the z-axis (the axis perpendicular to the screen). Elements with a higher z-index value will appear in front of elements with lower values.

Syntax:

.element {
    z-index: value;
}

Usage:

  1. Positive Values:

    • Elements with higher positive z-index values stack on top of elements with lower values.
    • The default z-index value is auto, which means elements stack in the order they appear in the HTML.

  2. Negative Values:

    • Elements with negative z-index values stack behind elements with positive values and elements with z-index: auto.
    • Negative z-index values can be useful for placing elements below the page's base layer.

  3. Stacking Contexts:

    • z-index only works on positioned elements (position: relative, position: absolute, or position: fixed).
    • Each positioned element with a non-auto z-index value creates a stacking context.
    • Descendant elements within a stacking context are stacked relative to that context.

Example:

.base {
    z-index: 1; /* Base layer */
    position: relative;
}
 
.overlay {
    z-index: 2; /* Overlay on top of the base layer */
    position: absolute;
    top: 0;
    left: 0;
}
 
.backdrop {
    z-index: -1; /* Behind the base layer */
    position: fixed;
    top: 0;
    left: 0;
}

Use Cases:

  • Modals and Overlays: Use higher z-index values for modals and overlays to ensure they appear above other content.
  • Sticky Headers and Footers: Use z-index to keep sticky headers and footers above the page content.
  • Layered Navigation: Control the stacking order of dropdown menus or tooltips in layered navigation menus.

Tips:

  • Avoid Excessive Use: Overuse of z-index can lead to stacking issues and make the codebase harder to maintain.
  • Be Mindful of Stacking Contexts: Understand how stacking contexts are formed and nested within each other to avoid unexpected results.

Conclusion:

Mastering the z-index property allows you to control the visual hierarchy and layering of elements on a web page. By understanding its syntax, usage, and best practices, you can create visually appealing and well-structured user interfaces.

8. Animations and Transitions10. Accessibility