body {
　　background: url("img_tree.png") no-repeat fixed 50% 50%/cover #ffffff ;
}

等價於

body {
　　background-image: url("img_tree.png");
　　background-repeat: no-repeat;
　　background-attachment: fixed;

　　background-position: 50% 50%; 　
　　background-size: cover; 　　
　　background-color:#fff;
 }

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