CSS

header center

div {
  width: 100%;
  height: 200px;
  display: flex;
  justify-content: center;
  align-items: center;
  background: blue;
}

text {
  background: orange;
}

<div>
   <text>Centered horizontally and vertically</text>
</div>

how to center a html header

h1{text-align: center;}