Answers for "center a div on a page vertically and horizontally"

center div horizontally and vertically

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

how to center a position fixed element horizontally

left: 50%;
margin-left: -400px; /* Half of the width */