CSS - z 索引

CSS z-index 属性

CSS z-index属性用于控制网页中元素在同一堆叠上下文中重叠时的堆叠顺序。z-index 值较高的元素出现在值较低的元素前面。

下图演示了 z-index 布局以供参考：

z 索引

z-index 属性可与嵌套在其他定位元素内部的定位元素一起使用。

句法

CSS 允许以多种方式设置元素的 z-index 属性。让我们检查所有可能的可用语法来设置元素的 z 索引。

关键词值

默认值。堆栈顺序与父元素的堆栈顺序相同。

z-index: auto;

整数值

正整数或负整数。它将元素的堆栈级别设置为给定值。较低的值将具有较低的优先级，并且元素将位于其他元素后面的一层。

z-index: 0;
z-index: 5;
z-index: 100;
z-index: -1;

适用于

所有定位元素。

DOM语法

object.style.zIndex = "2";

CSS z-index 自动

CSS z-index: auto将元素的 z-index 设置为其父元素的堆栈顺序。它是 z-index 属性的默认值。

例子

这是一个例子 -

<html>
<head>
<style>
   .box1 {
      position: absolute;
      height: 200px;
      width: 280px;
      background-color: #f0baba;
      z-index: auto;
      text-align: center;
      padding: 3px;
      left: 10px;
      top: 10px;
   }
   .box2 {
      position: absolute;
      height: 120px;
      width: 200px;
      background-color: #eae98f;
      z-index: 1;
      text-align: center;
      padding: 5px;
      margin: 20px;
      left: 30px;
      top: 30px;
   }
   p {
      margin-top: 250px;
   }
</style>
</head>
<body>
   <p>The element with z-index value of auto appears behind the element with the z-index value of 1.</p>
   <div class="box1">
      <span>CSS z-index: auto</span>
      <div class="box2">
         <span>CSS z-index: 1</span>
      </div>
   </div>

</body>
</html>

CSS z-index - 正整数

CSS z-index 属性可以有一个正整数值。在堆叠顺序中，具有较高整数值的元素将出现在具有较低值的元素之上。

例子

这是一个例子 -

<html>
<head>
<style>
   .box1 {
      position: absolute;
      height: 200px;
      width: 280px;
      background-color: #f0baba;
      z-index: 1;
      text-align: center;
      padding: 3px;
      left: 10px;
      top: 10px;
   }
   .box2 {
      position: absolute;
      height: 140px;
      width: 220px;
      background-color: #eae98f;
      z-index: 2;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 30px;
      top: 30px;
   }
   .box3 {
      position: absolute;
      height: 90px;
      width: 160px;
      background-color: #b7c8ae;
      z-index: 3;
      text-align: center;
      padding: 5px;
      margin: 20px;
      left: 50px;
      top: 50px;
   }
   p {
      margin-top: 250px;
   }
</style>
</head>
<body>
   <p>The element with z-index value of 1 appears behind the element with the z-index value of 2 and 3.</p>
   <div class="box1">
      CSS z-index: 1
   </div>
   <div class="box2">
      CSS z-index: 2
   </div>
   <div class="box3">
      CSS z-index: 3
   </div>
</body>
</html>

CSS z-index - 负整数

您还可以对 z-index 属性使用负整数值。具有负 z-index 值的元素将堆叠在具有较高 z-index 值的元素下方。

例子

这是一个例子 -

<html>
<head>
<style>
   .box1 {
      position: absolute;
      height: 200px;
      width: 280px;
      background-color: #f0baba;
      z-index: -3;
      text-align: center;
      padding: 3px;
      left: 10px;
      top: 10px;
   }
   .box2 {
      position: absolute;
      height: 140px;
      width: 220px;
      background-color: #eae98f;
      z-index: -2;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 30px;
      top: 30px;
   }
   .box3 {
      position: absolute;
      height: 90px;
      width: 160px;
      background-color: #b7c8ae;
      z-index: -1;
      text-align: center;
      padding: 5px;
      margin: 20px;
      left: 50px;
      top: 50px;
   }
   p {
      margin-top: 250px;
   }
</style>
</head>
<body>
   <p>The element with z-index value of -3 appears behind the element with the z-index value of -2 and -1.</p>
   <div class="box1">
      CSS z-index: -3
   </div>
   <div class="box2">
      CSS z-index: -2
   </div>
   <div class="box3">
      CSS z-index: -1
   </div>
</body>
</html>

CSS z-index 宽度位置：粘性

以下示例演示了如何使用z-index属性来控制具有position:sticky属性的元素的堆叠顺序，以便它们在页面滚动时保持固定位置 -

<html>
<head>
<style>
   .box1 {
      position: sticky;
      height: 200px;
      width: 280px;
      background-color: #f0baba;
      z-index: 1;
      text-align: center;
      padding: 3px;
      margin: 10px;
      left: 10px;
      top: 80px;
   }
   .box2 {
      position: sticky;
      height: 140px;
      width: 220px;
      background-color: #eae98f;
      z-index: 2;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 40px;
      top: 200px;
   }
   .box3 {
      position: sticky;
      height: 90px;
      width: 160px;
      background-color: #b7c8ae;
      z-index: 3;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 70px;
   }
</style>
</head>
<body>
   <p>Move cursor upward to see the effect.</p>
   <div class="box1">
      CSS z-index: 1
   </div>
   <div class="box2">
      CSS z-index: 2
   </div>
   <div class="box3">
      CSS z-index: 3
   </div>
</body>
</html>

CSS z-index 宽度位置：固定

以下示例演示了如何使用z-index属性使元素在用户向下滚动时保持在内容顶部，即使它具有position :fixed属性 -

<html>
<head>
<style>
   .container {
      position: relative;
      height: 350px;
   }
   .box1 {
      position: fixed;
      height: 200px;
      width: 280px;
      background-color: #f0baba;
      z-index: -3;
      text-align: center;
      padding: 3px;
      left: 10px;
      top: 10px;
   }
   .box2 {
      position: fixed;
      height: 140px;
      width: 220px;
      background-color: #eae98f;
      z-index: -2;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 30px;
      top: 30px;
   }
   .box3 {
      position: fixed;
      height: 90px;
      width: 160px;
      background-color: #b7c8ae;
      z-index: -1;
      text-align: center;
      padding: 5px;
      margin: 20px;
      left: 50px;
      top: 50px;
   }
   h3 {
         margin-top: 320px;
   }
</style>
</head>
<body>
   <h3>Scroll down the content to see the effect.</h3>
   <div class="container">
      <div class="box1">
         CSS z-index: -3
      </div>
      <div class="box2">
         CSS z-index: -2
      </div>
      <div class="box3">
         CSS z-index: -1
      </div>
   </div>
</body>
</html>

CSS z-index 宽度位置：静态

以下示例显示z-index属性不会影响具有position: static属性的元素的堆叠顺序 -

<html>
<head>
<style>
   .box1 {
      position: static;
      height: 200px;
      width: 280px;
      background-color: #f0baba;
      z-index: 1;
      text-align: center;
      padding: 3px;
      margin: 10px;
      left: 10px;
      top: 10px;
   }
   .box2 {
      position: static;
      height: 140px;
      width: 220px;
      background-color: #eae98f;
      z-index: 2;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 30px;
      top: 30px;
   }
   .box3 {
      position: static;
      height: 90px;
      width: 160px;
      background-color: #b7c8ae;
      z-index: 3;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 50px;
      top: 50px;
   }
</style>
</head>
<body>
   <p>The z-index property has no effect on the stacking order of elements if the position property is set to static.</p>
   <div class="box1">
      CSS z-index: 1
   </div>
   <div class="box2">
      CSS z-index: 2
   </div>
   <div class="box3">
      CSS z-index: 3
   </div>
</body>
</html>

CSS z-index 宽度位置：相对

该示例显示，当元素具有position:relative属性时，z-index属性相对于其在文档流中的原始位置定位元素。

<html>
<head>
<style>
   .box1 {
      position: relative;
      height: 200px;
      width: 280px;
      background-color: #f0baba;
      z-index: 1;
      text-align: center;
      padding: 3px;
      margin: 10px;
      left: 10px;
      top: 10px;
   }
   .box2 {
      position: relative;
      height: 140px;
      width: 220px;
      background-color: #eae98f;
      z-index: 2;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 30px;
      top: 30px;
   }
   .box3 {
      position: relative;
      height: 90px;
      width: 160px;
      background-color: #b7c8ae;
      z-index: 3;
      text-align: center;
      padding: 5px;
      margin: 10px;
      left: 50px;
      top: 50px;
   }
</style>
</head>
<body>
   <p>The z-index property positions the element relative to its original position if position is relative.</p>
   <div class="box1">
      CSS z-index: 1
   </div>
   <div class="box2">
      CSS z-index: 2
   </div>
   <div class="box3">
      CSS z-index: 3
   </div>
</body>
</html>