While you should generally use CSS for transitions as much as possible, there are some effects that can't be achieved without JavaScript, such as a typewriter effect:

function typewriter(node, { speed = 1 }) {
  const valid = (
    node.childNodes.length === 1 &&
    node.childNodes[0].nodeType === Node.TEXT_NODE
  );

  if (!valid) {
    throw new Error(`This transition only works on elements with a single text node child`);
  }

  const text = node.textContent;
  const duration = text.length / (speed * 0.01);

  return {
    duration,
    tick: t => {
      const i = Math.trunc(text.length * t);
      node.textContent = text.slice(0, i);
    }
  };
}