I have a div with adding a class by condition

        <div
          className={`${
            showLinks ? "links-container show-container" : "links-container"
          }`}
        >

And some styles

.links-container {
  height: 0;
  overflow: hidden;
  transition: all 0.3s linear;
}
.show-container {
  height: 10rem;
}

So when the component is render/rerender, a smooth div extension animation is triggered thanks to my transition style. How it works? Is this css or react rules?