<body>
  <small>This text is kinda small.</small>
  <h3>How about this?</h3>
  <h2>Better. But I know we can do more.</h2>
  <h1>Now that's what I'm talking about.</h1>
  <div id="fontSizeIndicator"></div>
</body>
body {
  text-align: center;
  margin-top: 2em;
  font-family: apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans,
    Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
}

#fontSizeIndicator {
  position: fixed;
  font-size: 1.2rem;
  background: green;
  color: white;
  box-shadow: 0 0 12px rgba(0, 0, 0, 0.5);
  top: .5em;
  left: .5em;
  display: inline-block;
  padding: .5em;
  border: 2px solid white;
  border-radius: 0.5em;
  &::before {
    content: "Font size: ";
  }
}

h1 {
  font-size: 4rem;
}
h3 {
  font-size: 1.5rem;
}
h2 {
  font-size: 2rem;
}
View Compiled 
document.addEventListener("selectionchange", (e) => {
  const sel = window.getSelection();
  if (sel.rangeCount) {
    const indicator = document.getElementById("fontSizeIndicator");
    indicator.innerText = window.getComputedStyle(
      sel.getRangeAt(0).startContainer.parentNode
    ).fontSize;
  }
});
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