<!DOCTYPE html>
<html>
<head>
<style>
.flexbox {
    display: flex;
}
.flex-item {
    width: 50px;
    height: 50px;
    margin-top: 10px;
    background-color: green;
}
</style>
</head>
<body>
    <div class="flexbox">
        <div id="flex-item" class="flex-item"></div>
    </div>
</body>
</html>
