how to set position in middile in css code example

Example 1: 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>

Example 2: css position element in the middle of a header

P.blocktext {
    margin-left: auto;
    margin-right: auto;
    width: 200px;/*usually works best with a fixed width*/
}