Везувий
Сталин 
<html>
<head>
<title>Testing Css</title>
<meta content="width=device-width,initial-scale=1" name="viewport">
</head>
<style>
body {
background: grey;
margin: auto;
max-width: 420px;
}
.flex {
display: flex;
-webkit-align-items: center;
-webkit-box-align: center;
-ms-flex-align: center;
align-items: center;
}
.flex > div {
width: 50%
}
.stl {
border: 2px solid #303030;
height: 42px;
text-align: center;
margin: 1px;
}
</style>
<body>
<div class="flex">
<div class="stl">
hdjdjdhdh
</div>
<div class="stl">
jdjdjd
</div>
</div>
</body>
<html>
So4nOFF 
display: block;
margin-left: auto;
margin-right: auto;