.center {
  /* make children flex-items */
  display: inline-flex;
  /* align-items in column */
  flex-direction: column;
  /* center horizontally */
  align-items: center;
  /* center vertically */
  justify-content: center;
  
  width: 100%;
  height: 100%;
}

html, body {
  background: YELLOW;
  margin: 0;
  padding: 0;
}

h1{
  display: block;
  margin-block-start: 0;
  margin-block-end: 0;
  font-size: 3em;
  text-align: center;
  font-weight: bold;
  unicode-bidi: isolate;
}