<div class="container">
  <h1>font-size: <span id="val">100</span>%</h1>
</div>

<div class="actions">
  <label for="percentage">font-size:</label>
  <input type="range" id="percentage" name="percentage" min="10" max="300" value="100" step="5">
  <span>%</span>
</div>

@import url("https://fonts.googleapis.com/css?family=Gochi+Hand");

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

body {
  width: 100vw;
  min-height: 100vh;
  margin: 0;
  background-color: #291642;
  font-family: "Gochi Hand", sans-serif;
  font-size: 100%;
  letter-spacing: 0.1rem;
  color: #fff;
  display: flex;
  flex-direction: column;
  align-items: center;

  background-blend-mode: hue;
}

.container {
  flex: 1;
  min-height: 0;
  width: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
  position: relative;
}

.actions {
  padding: 3vh;
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: rgba(0, 0, 0, 0.5);
  width: 100%;
  font-size: 2rem;
}

:root {
  --font-size: 100%;
}

h1 {
  display: inline-flex;
  font-size: var(--font-size);
  background: linear-gradient(to right, #44ea76 0%, #39fad7 80%, #39fad7 100%)
      no-repeat,
    url("https://s3-us-west-2.amazonaws.com/s.cdpn.io/142996/slider-2.jpg")
      no-repeat fixed;
  background-size: cover;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}

const rangeElement = document.getElementById("percentage");
const spanElement = document.getElementById("val");

rangeElement.addEventListener("change", (etv) => {
  document.documentElement.style.setProperty(
    "--font-size",
    `${etv.target.value}%`
  );

  val.textContent = etv.target.value;
});