我想瓷砖，通过它我想打印整个Web文档的图像。我已经使用背景属性瓦，但我必须设置的z-index属性，以使该图像放置在网页中的所有元素。帮我解决这个问题。
坚持到根的风格
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
:root
{
background-image:url("https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQRxExzbsZCvO6lpE1tll_7pQLB5nd4RkWkrMannssfhAYni_Ct");
opacity: .4;
}
</style>
</head>
<body>
<div id="overlay" onclick="off()"></div>
<div style="padding:20px">
<h2>Overlay</h2>
<p>Add an overlay effect to the page content (100% width and height with background with 40% opacity).</p>
<button>some button here</button>
</div>
</body>
</html>