how to make background image correct resolution in html code example

Example 1: how to set a full size background image in html

<head>
<style>
#example1{
  border: 2px solid black;
  padding: 100px;
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: 100% 100%;
}
</style>
</head>
<body>
<div id="example1">
</div>
</body>

Example 2: how to set background automatically with my screen height

body {

    background-image: url("img_tree.gif");

   background-repeat: no-repeat;   
 background-attachment: fixed;
  
   background-position: top center;
  background-size: contain;

 }

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Css Example