Answers for "centering a div vertically"

css align items vertical center

.parent {
  display: flex;
  justify-content: center;
  align-items: center;
}

center elements vertically in div

.container {        
    display: flex;
    align-items: center;
}

css vertical center

/* No Flexbox */
.parent {
  position: relative;
}
.child {
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
}

/* With Flexbox */

.parent {
  display: flex;
  flex-direction: column;
  justify-content: center;
}

css center vertically

/* 100vh = 100% Viewport Height */
body {
	display: flex;
	height: 100vh;
	flex-direction: column;
	justify-content: center;
}
/* add */ align-items: center; /*to also center horizontally.*/

css center vertically

<style>
.container {
  height: 200px;
  position: relative;
  border: 3px solid green;
}

.center {
  margin: 0;
  position: absolute;
  top: 50%;
  left: 50%;
  -ms-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
}
</style>

<div class="container">
  <div class="center">
    <p>I am vertically and horizontally centered.</p>
  </div>
</div>