<main>
  <p id="pg">
    これは Codepen のテストです
  </p>
  <button id="button">
    ボタン
  </button>
</main>

main p {
  font-size: 2rem;
  font-weight: 700;
  text-decoration: underline;
}

const btn = document.getElementById("button");
const pg = document.getElementById("pg");

btn.addEventListener("click", () => {
  pg.style.color = "red";
});