<!DOCTYPE html>
<html>
<head>
<title>Title of the document</title>
<style>
.container {
display: flex;
align-items: center;
background-color: #f8fc03;
height: 200px;
}
</style>
</head>
<body>
<div class="container">
<div>Hello world!</div>
</div>
</body>
</html>