rotateX()

transform: rotateX(0);

transform: rotateX(45deg);

transform: rotateX(-0.2turn);

transform: rotateX(3.142rad);

<section id="default-example">
  <img
    class="transition-all"
    id="example-element"
    src="/shared-assets/images/examples/firefox-logo.svg"
    width="200" />
</section>

rotateX(a)

<div>Normal</div>
<div class="rotated">Rotated</div>

div {
  width: 80px;
  height: 80px;
  background-color: skyblue;
}
.rotated {
  transform: rotateX(45deg);
  background-color: pink;
}