HTML Code

<!DOCTYPE html>

<html>

<head>

  <title>CSS :root Responsive Example</title>

  <style>

    :root {

      --font-size: 1rem;

    }



    @media (min-width: 768px) {

      :root {

        --font-size: 1.125rem;

      }

    }



    body {

      font-family: system-ui, sans-serif;

      padding: 1.5rem;

      font-size: var(--font-size);

      color: #334155;

      line-height: 1.6;

    }

  </style>

</head>

<body>

  <p>Resize the window: body text is 16px on small screens and 18px from 768px upward.</p>

</body>

</html>

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CSS :root Selector - Responsive Font Size