Center div css

.center {
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

Center with css

.parent {
  position: relative;
}
.child {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

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

.parent {
  position: relative;
}
.child {
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

IMG.displayed {
    display: block;
    margin-left: auto;
    margin-right: auto }
 ...
<IMG class="displayed" src="..." alt="...">

Source: w3.org

.container {
  display: grid;
  place-items: center;
}

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