"Center a div" is the classic CSS warm-up. The modern answer is Flexbox or Grid; know a couple of others and why they differ.

Flexbox (the go-to)

.parent {
  display: flex;
  justify-content: center;  /* main axis — horizontal by default */
  align-items: center;      /* cross axis — vertical */
}

Works regardless of the child's size, content, or count. The everyday answer.

Grid (the shortest)

.parent {
  display: grid;
  place-items: center;      /* centers on both axes in one line */
}

place-items: center is the most concise modern centering.

Absolute + transform (when you can't use fl/grid)

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

The top/left: 50% positions the child's top-left corner at the center; translate(-50%, -50%) pulls it back by half its own size — so it works without knowing the child's dimensions. Useful for overlays/modals.

margin: auto inside flex/grid

.parent { display: flex; }
.child  { margin: auto; }   /* auto margins absorb space on all sides */

In a flex/grid container, margin: auto centers on both axes. (Plain margin: 0 auto only centers horizontally and only for block elements with a defined width.)

What interviewers are really checking

• Do you reach for Flexbox/Grid (modern) or old hacks?
• Do you know horizontal-only (margin: 0 auto, text-align: center for inline content) vs both axes?
• Do you know which methods need the element's size known vs not? (transform method doesn't; negative-margin method does.)

The framing

"Modern answer: Flexbox on the parent — justify-content: center and align-items: center — or Grid with place-items: center, which is the shortest. Both work without knowing the child's size. When I can't restructure the parent, absolute positioning with top/left: 50% plus transform: translate(-50%, -50%) centers it relative to its own size — good for modals. And margin: auto inside a flex container also does both axes. The thing to be precise about is horizontal-only versus both, and which methods need the element's dimensions."