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HomeWeb Front-endHTML Tutorial认识CSS中的z-index元素层级属性_html/css_WEB-ITnose

Z-index:指定了元素及其子元素的Z轴顺序,在Z元素发生覆盖的时候,哪个在下面,哪个在上面,一般都由Z-index来决定。Z-index的支持的属性值有:z-index:auto/整数/inherit;基本特性有:支持负值,支持animation动画,在css2.1中,需要和定位元素配合使用;如果不考虑CSS3,z-index只对定位元素起作用;


z-index的使用:

    1、如果定位元素不发生嵌套,即是同级元素 ,则遵循“后来居上”和“大小”原则;

    2、如果发生嵌套,则遵循“祖先优先”原则,由父级元素的z-index决定,前提是父级元素的z-index值为数值,此时会忽略子代元素的z-index值;

css中的层叠上下文和层级水平:层叠上下文简单的说就是一个包含了一组堆叠层的元素,它们在Z轴上有着特定的顺序,页面根元素和由z-index为数值的定位元素具有层叠上下文;

层级水平:它决定了同一个层叠上下文中的元素在Z轴上的显示顺序,和z-index并不同;

层叠上下文的特点:可以嵌套,组成分层次的层叠上下文;每个层叠上下文和兄弟元素独立,当进行层叠变化时,只影响子元素;


著名的7阶层叠水平:

小解答:1、为什么inline/liline-block的层级水平大于float的层级水平?

因为行内元素一般承载的是内容,为了符合页面加载的美观,所以行内元素会覆盖浮动元素;

2、为什么定位元素会覆盖普通元素?

因为定位元素z-index:0;而普通元素为行内元素或浮动元素,根据7阶层叠水平表,普通元素和浮动元素都会被覆盖;

3:z-index:0为什么不等于z-index:auto?

因为z-index:0;会创建层叠上下文,而z-index:auto;不会,但他们在层叠顺序上是一样的;

其他CSS属性影响层叠上下文的层叠顺序:

1、z-index的值不为auto的flex项;

2、元素的opacity不是1;

3、元素的transform不是none;

4、元素的mix-blend-mode的值不是normal;

5、元素的filter值不是none;

6、元素的position:fixed声明;

7、移动端的webkit-overflow-scrolling设为otuch;

8、will-change指定的属性中的任何一个;



使用z-index需要注意的问题:

1、最小化原则;为了避免z-index嵌套层叠关系混乱,尽量避免使用定位属性;

2、“不超2”原则:非浮层元素,避免设置z-index值超过2,一般在0,1,2;可以通过调节DOM节点的书序来说实现;

3、浮层组件计数器:为了避免浮层组件被z-index值高 的元素覆盖,通过JS获取body下子元素层级最高数,将浮层的z-index值设为+1;

4:可访问性隐藏:z-index:-1;



参考:http://www.zhangxinxu.com/wordpress/?p=5115





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