Interpret the z-index property of CSS in cascading layout

Detailed explanation of the usage of z-index attribute in CSS in cascading layout

In web development, it is often necessary to cascading layout of elements to achieve cascading layout between elements. coverage effect. The z-index property in CSS is used to control the stacking order of elements. This article will introduce the use of z-index attributes in cascading layout in detail and provide specific code examples.

1. The basic concept of z-index attribute

The z-index attribute is used to specify the stacking order of elements in a cascading layout. The value is an integer. The larger the value, the closer the element is to the front, that is, it is displayed on the upper level. If the z-index values of two elements are the same or the z-index attribute is not set, the stacking order is determined based on their order in the HTML code. The z-index attribute can only be applied to elements whose position attribute value is relative, absolute, or fixed, and is invalid for elements with other position attribute values (such as static).

2. Usage of z-index attribute

1. Z-index attribute of a single element
   By setting the z-index attribute of a single element, the element can be placed in a cascading layout display effect. Such as the following sample code:

    
  
Box 1
 
Box 2
 
Box 3
  

In the above code, box1, box2 and box3 respectively represent three elements with different z-index values. Box3 has the largest z-index value, so it will be on the top layer in the stacked layout, box2 is in the middle, and box1 is on the bottom layer.

2. Z-index attribute of child elements
   When both the parent element and the child element have the z-index attribute set, the z-index value of the parent element will not affect the child element in the cascading layout. display effect. The z-index of child elements will still take effect in child elements of the same level. Such as the following sample code:

    
  
 
Child 1
 
Child 2
 
  

In the above code, parent represents the parent element, and child1 and child2 represent two child elements. Although the parent element sets the z-index value, it has no effect on the stacking order of the child elements. child2 still has the largest z-index value, so it will be on top in the stacked layout.

3. Notes on the z-index attribute

1. The z-index attribute is only valid for positioned elements
   When using the z-index attribute, you need to ensure the position attribute of the element Values are relative, absolute, or fixed. For other position attribute values, such as static, the default stacking order will be determined based on the order of elements in the HTML code.
2. The z-index attribute only takes effect inside the parent element
   When both the parent element and the child element are set with the z-index attribute, the stacking order of the child elements only takes effect inside the parent element. If the z-index values of two parent elements conflict with each other, the stacking order of child elements may not be displayed properly.

4. Summary

The z-index attribute plays an important role in the cascading layout in CSS. You can control the cascading order of elements by setting the z-index value. Through the sample code provided in this article, readers can better understand and apply the z-index attribute to achieve coverage effects between various elements.

It should be noted that the z-index attribute is only valid for positioned elements and takes effect within the parent element. In actual development, setting the z-index value reasonably according to actual needs can achieve a more elegant and hierarchical page layout effect.

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