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<!DOCTYPE html>
<html>
<head>
<style> 
.box {
  background-color: lightgreen;
  height: 90px;
  width: 90px;
}
.box:empty {
  background-color: salmon;
}
</style>
</head>
<body>
<h1>Demo of :empty</h1>
<div class="box"></div><br>
<div class="box">I will be lightgreen.</div><br>
<div class="box"><!-- I will be salmon --></div><br>
<div class="box"> <!-- I will be lightgreen --> </div>
</body>
</html>