Z-index is a CSS property that controls the vertical order of elements on a web page. It decides which element appears in front and which one stays behind when elements overlap each other.
Understanding Stacking
When multiple elements are placed on a page, they are stacked one above another. By default, elements follow the order in which they appear in the HTML. Elements written later usually appear on top of earlier ones.
What is Z-index
Z-index allows you to control this stacking order manually. It works only on elements that have a position value such as relative absolute or fixed.
A higher z index value means the element will appear in front of elements with lower values.
Example of Z-index
div one is positioned with z index 1
div two is positioned with z index 2
In this case div two will appear above div one because it has a higher z index value.
Position Requirement
Z-index only works when an element has a position property set to relative absolute fixed or sticky. If position is not set, z index will not affect the element.
Stacking Context
A stacking context is a group of elements that are stacked together. When a new stacking context is created, the z index values inside it only work within that context.
Some properties that create a new stacking context include position with a z index value opacity less than one and transform.
Common Use Cases
Z-index is often used for navigation menus dropdowns modals tooltips and overlapping images. It helps ensure that important elements stay visible above others.
Best Practices
Use small and meaningful z index values instead of very large numbers
Keep your stacking order organized and consistent
Avoid unnecessary stacking contexts to prevent confusion
Test overlapping elements to ensure correct display
Conclusion
Z-index is a powerful tool for controlling how elements overlap on a webpage. By understanding stacking and positioning you can design clean and professional layouts with proper layering of content.