<!DOCTYPE html>
<html>
<head>
<title>Title of the document</title>
<style>
#boxThis {
padding: 5px;
background-color: #80c294;
font-size: 1.5em;
width: 300px;
text-align: center;
font-weight: bold;
border: 1px solid #666;
-webkit-border-radius: 10px;
border-radius: 10px;
}
#boxThis.box {
margin-top: 0;
position: fixed;
top: 0;
z-index: 9999;
-webkit-border-radius: 0 0 10px 10px;
border-radius: 0 0 10px 10px;
}
</style>
<script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
</head>
<body>
lorem
<br /> lorem
<br /> lorem
<br /> lorem
<br /> lorem
<br /> lorem
<br /> lorem
<br /> lorem