<div class="container">
  <div class="box">font-size: 100%(16px)</div>
  <h1>font-size: <span id="val">100</span>% (<strong>16px</strong>) <i id="line-height">line-height: 120% (19.2px)</i></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%;
  line-height: 150%;
  letter-spacing: 0.1rem;
  color: #fff;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  background-blend-mode: hue;
}

body::before {
  content: "<body> ❯ line-height: 150% x 16px = 24px";
}

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

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

: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;
}

.container {
  width: 50vmax;
  height: 50vmin;
  border: 1px dashed #f36;
  position: relative;
  font-size: var(--font-size);
}

.box {
  background-color: #f36;
  position: absolute;
  right: 0;
  bottom: 100%;
  padding: 4px 6px;
  font-size: 16px;
  border-radius: 4px 4px 0 0;
  font-size: 16px;
}

.container::before {
  content: '<div class="container"></div> | 基本字号: 16px';
  position: absolute;
  left: 0;
  bottom: 100%;
  font-size: 16px;
}

h1 {
  border: 1px dotted #0ff;
  padding: 20px 50px;
  border-radius: 5px;
  position: relative;
  line-height: 120%;
}

h1::before {
  content: "<h1>";
  position: absolute;
  left: 0;
  bottom: 100%;
  color: #fff;
  -webkit-text-fill-color: #fff;
  z-index: 2;
  font-size: 16px;
  line-height: 1.15;
}

h1 i {
  position: absolute;
  left: 50%;
  top: 100%;
  transform: translate(-50%, 0);
  -webkit-text-fill-color: #fff;
  white-space: nowrap;
}

const rangeElement = document.getElementById("percentage");
const spanElement = document.getElementById("val");
const boxElement = document.querySelector(".box");
const strongElement = document.querySelector("strong");
const iElement = document.getElementById("line-height");

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

  val.textContent = etv.target.value;

  const containerComputedVal = (etv.target.value * 16) / 100;

  boxElement.textContent = `font-size:${etv.target.value}% (${containerComputedVal}px) || line-height: 24px`;

  const strontComputedVal =
    (((etv.target.value * 16) / 100) * etv.target.value) / 100;

  strongElement.textContent = `${strontComputedVal}px`;

  const lineHeightValue =
    (((((etv.target.value * 16) / 100) * etv.target.value) / 100) * 120) / 100;

  iElement.textContent = `line-height: 120% (${lineHeightValue}px)`;
});

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.