<p class="support">I'm sorry! Your browser does not support the <code>:is()</code> selector</p>
<h1>Headline</h1>

@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100vw;
  min-height: 100vh;
  font-family: "Exo", Arial, sans-serif;
  background-color: #557;
  color: #fff;
  display: grid;
  gap: 20px;
  place-content: center;
  font-size: clamp(1.5rem, 2vw + 1rem, 2.25rem);
  line-height: 1.5;
}

.support {
  color: #842029;
  background-color: #f8d7da;
  padding: 1rem 1rem;
  border: 1px solid #f5c2c7;
  border-radius: 0.25rem;
  font-size: 1rem;
}

@supports selector(:is(h1)) {
  :is(h1, h2, h3) {
    color: red;
  }

  .support {
    display: none;
  }
}