<!doctype html>
<meta charset="utf-8">
<style>
.flex {
display: flex;
width: 100px;
border: 1px solid;
}
</style>
<div class="flex">
<div>
<div style="background: green; width: 150px; height: 50px;"></div>
</div>
</div>
<!doctype html>
<meta charset="utf-8">
<style>
.flex {
display: flex;
width: 100px;
border: 1px solid;
}
</style>
<div class="flex">
<div>
<div style="background: green; width: 150px; height: 50px;"></div>
</div>
</div>