Containment and Rendering Performance

What Is It?

Containment limits how a region interacts with the rest of a page. Content visibility can let the browser skip offscreen rendering work while preserving the content in the document.

Properties, Tags and Attributes Explained

• contain: Applies layout, paint, size, or style containment, with different effects for each.
• content-visibility: auto: Lets the browser skip rendering work for irrelevant offscreen content.
• contain-intrinsic-size: Provides placeholder size information when size containment applies.
• will-change: Hints at expected changes; excessive hints can waste memory and resources.

Each example is a complete HTML page. The CSS is inside the head's style element. Use Try in new tab to edit the CSS or HTML, run your changes, and reset the example.

Example 1: Paint containment

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Containment and Rendering Performance</title>
  <style>
    .demo { contain: paint; width: 180px; padding: 16px; border: 2px solid #64748b; }
    .child { width: 260px; background: #dbeafe; }
  </style>
</head>
<body>
  <div class="demo"><div class="child">This child paints only inside the containing boundary.</div></div>
</body>
</html>

What you see: The oversized child is clipped at the paint containment boundary.

Example 2: Offscreen sections

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Containment and Rendering Performance</title>
  <style>
    .lesson { content-visibility: auto; contain-intrinsic-size: auto 220px; padding: 20px; border: 1px solid #cbd5e1; margin-bottom: 12px; }
  </style>
</head>
<body>
  <section class="lesson"><h2>First section</h2><p>Read the first lesson.</p></section><section class="lesson"><h2>Second section</h2><p>Offscreen sections may defer rendering.</p></section>
</body>
</html>

What you see: The sections remain readable. Performance effects become measurable with many offscreen sections, rather than a tiny example.

Example 3: Apply a limited rendering hint

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Containment and Rendering Performance</title>
  <style>
    .tile { display: inline-block; padding: 20px; background: #dbeafe; }
    .tile:hover, .tile:focus-visible { will-change: transform; transform: translateY(-2px); }
    @media (prefers-reduced-motion: reduce) { .tile:hover, .tile:focus-visible { transform: none; will-change: auto; } }
  </style>
</head>
<body>
  <button class="tile" type="button">Inspect the hint in DevTools</button>
</body>
</html>

What you see: The hint is present only during the interactive state. Its performance benefit is not visible in this tiny demo; measure a real rendering problem before using it, and prefer no hint by default.

When and Where to Use It

Consider these tools for long feeds and isolated widgets after measuring a rendering problem.

Common Mistake or Useful Tip

Containment changes layout or clipping behavior. Do not add it everywhere as an automatic optimization, and remove will-change after a temporary need ends.

Try It Yourself

Remove contain: paint and observe where the oversized child paints.