HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>scrollsnapchange basic</title>
  <style>
    #snap-box {
      width: 220px; height: 160px; overflow-y: scroll;
      border: 2px solid #333; scroll-snap-type: y mandatory;
    }
    #snap-box section {
      height: 160px; scroll-snap-align: start;
      display: flex; align-items: center; justify-content: center;
      border-bottom: 1px solid #ccc; font: 16px sans-serif;
    }
    #snap-box section.selected { outline: 3px solid #0a7; background: #e8fff4; }
    #out { margin-top: 10px; font: 14px sans-serif; }
  </style>
</head>
<body>
  <div id="snap-box">
    <section data-label="Card 1">Card 1 — scroll down</section>
    <section data-label="Card 2">Card 2</section>
    <section data-label="Card 3">Card 3</section>
  </div>
  <p id="out">Scroll until a card snaps (Chromium 129+)</p>
  <script>
    var box = document.getElementById("snap-box");
    var out = document.getElementById("out");
    box.addEventListener("scrollsnapchange", function (event) {
      if (event.snapTargetBlock) {
        event.snapTargetBlock.classList.add("selected");
        out.textContent = "selected: " + event.snapTargetBlock.dataset.label;
      }
    });
  </script>
</body>
</html>

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Element scrollsnapchange — mark selected (Try It)