How can I center a div horizontally and vertically?

You can center a div using CSS Flexbox or CSS Grid. Here are examples for both methods:

/* Flexbox method */
.container {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
}
/* Grid method */
.container {
    display: grid;
    place-items: center;
    height: 100vh;
}

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