Ways to center elements in CSS

centering.css

/* Grid/Flexbox */
.parent {
  display: flex; /*grid*/
  justify-content: center;
  align-items: center;
}

/* Grid/Flexbox with margin */
.parent {
  display: flex; /*grid*/
}
.child {
  margin: auto;
}

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

/* Known size */
.parent {
  position: relative;
}
.child {
  position: absolute;
  --height: 100px;
  --width: 200px;
  height: var(--height);
  width: var(--width);
  top: calc(50% - var(--height) / 2);
  left: calc(50% - var(--width) / 2);
}

/* Center inline content of table cell */
table td {
  text-align: center;
  vertical-align: middle;
}

/* Center label of a button */
.button {
  --height: 30px;
  height: var(--height);
  line-height: var(--height);
  text-align: center;
}

/* Center bg image */
.has-background {
  background-image: 'url(...)';
  background-repeat: no-repeat;
  background-position: center;
  background-size: 16px;
  background-color: blue;
}