CSS, center elements with varying width and height

Example markup where the width or height can change or is unknown:

<main>
    <div class="item-centered">
      <p>This content length may change based on user input.</p>
    </div>
</main>

Using CSS3 transforms and the translate property:

main {
   position: relative;
}

// Vertically centered only
.item-centered {
    position: absolute; // can also be relative
    top: 50%;
    transform: translateY(-50%); // left out prefixes for brevity
}

// Horizontally centered only
.item-centered {
    position: absolute;
    left 50%;
    transform: translateX(-50%);
}

// Horizontally and vertically centered
.item-centered {
    position: absolute;
    left 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

Example:

Codepen