Why is my element with position:absolute always showing up underneath position:relative items?

问题

It's all in the question but here's a simple example

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" />
    <title>TEST</title>
</head>
<body style="margin:0;padding:0">

<div style="background-color:#eeeeee;margin:auto;height:500px;width:500px">
    <div style="position:relative">
        <span style="position:absolute;display:block;height:250px;width:250px;background:green;z-layer:2"><br /><br />Should be on top</span>
    </div>
    <span style="position:relative;display:block;width:500px;background:blue;z-layer:1">Actually on top</span>
</div>

</body>
</html>

回答1:

instead of "z-layer" use "z-index"

also the absolute span is in a relative div with no z-index

Here is the correct html:

<div style="background-color:#eeeeee;margin:auto;height:500px;width:500px">
    <div style="position:relative;z-index:2">
        <span style="position:absolute;display:block;height:250px;width:250px;background:green"><br /><br />Should be on top</span>
    </div>
    <span style="position:relative;display:block;width:500px;background:blue;z-index:1">Actually on top</span>
</div>

回答2:

It happens because when you position an element "absolutely" it is removed from the flow of the document in the Document Object Model and so elements that remain in the flow of the document appear "above" the removed element. For cross-browser compatibility place your z-index adjustments on the parent element of the absolutely positioned element.

来源：https://stackoverflow.com/questions/578554/why-is-my-element-with-positionabsolute-always-showing-up-underneath-positionr

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