W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
  font-family: Arial;
}

* {
  box-sizing: border-box;
}

.dot {
  height: 12px;
  width: 12px;
  background-color: #bbb;
  border-radius: 50%;
  display: inline-block;
}

.container {
  border: 3px solid #f1f1f1;
  border-top-left-radius: 4px;
  border-top-right-radius: 4px;
}

.top {
  padding: 10px;
  background: #f1f1f1;
  border-top-left-radius: 4px;
  border-top-right-radius: 4px;
}

.content {
  padding: 10px;
}
</style>
</head>
<body>

<div class="content">
    <h3>Browser Window</h3>
    <p>How to create a browser window look with CSS.</p>
  </div>
</div>

</body>

</html>

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<!DOCTYPE html><html><head><meta name="viewport" content="width=device-width, initial-scale=1"><style>body {  font-family: Arial;}​* {  box-sizing: border-box;}​.dot {  height: 12px;  width: 12px;  background-color: #bbb;  border-radius: 50%;  display: inline-block;}​.container {  border: 3px solid #f1f1f1;  border-top-left-radius: 4px;  border-top-right-radius: 4px;}​.top {  padding: 10px;  background: #f1f1f1;  border-top-left-radius: 4px;  border-top-right-radius: 4px;}​.content {  padding: 10px;}</style></head><body>​<div class="container">  <div class="top">    <span class="dot"></span>    <span class="dot"></span>    <span class="dot"></span>  </div>