Centring an element

The simplest way to centre an element on both axes is Grid or Flexbox.

.container {
  display: grid;
  place-items: center;
  min-height: 240px;
}

Alternative: display: flex; justify-content: center; align-items: center.

To be precise: the container needs free space on the axis where you want to see the centring. For a block with a set width, margin-inline: auto centres it horizontally.

Pitfall: text-align: center aligns inline content; it isn't a general way to centre any block vertically.