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Table of Contents
What does the contain property do?
When should you use containment?
How does containment improve performance?
Home Web Front-end CSS Tutorial What is CSS containment (contain property)?

What is CSS containment (contain property)?

Jun 24, 2025 am 12:34 AM

CSS containment improves performance by isolating elements to limit browser recalculations. 1. The contain property enables developers to specify how isolated an element is from the rest of the page. 2. Values like size, layout, paint, and style offer different levels of containment. 3. strict applies all types of containment, while content excludes size. 4. It's best used on self-contained components like widgets or cards. 5. Performance improvements come from limiting layout, painting, and rendering checks to only the contained element. 6. Avoid overuse as it may cause memory overhead. 7. Browser support varies across containment types, with layout and paint being more widely supported.

What is CSS containment (contain property)?

CSS containment is a feature that helps improve performance by limiting how much of the page the browser has to re-calculate when something changes. It’s especially useful on complex or large web pages where small updates shouldn’t require the whole page to be re-rendered.

The key idea behind containment is: tell the browser what parts of the page can be treated as independent, so it doesn’t waste time checking unrelated areas when making layout, style, paint, or size calculations.

This is done using the contain property in CSS.


What does the contain property do?

The contain property tells the browser how much a specific element should be isolated from the rest of the page. By doing this, it gives the browser hints about which parts of the page don’t need to be affected when something inside the contained element changes.

You might use it like this:

.widget {
  contain: strict;
}

Or more specifically:

.widget {
  contain: layout paint size;
}

Each value controls a different kind of isolation:

  • size – The element’s dimensions are independent of its children.
  • layout – Layout inside the element won’t affect anything outside of it.
  • paint – Nothing painted inside the element will appear outside it.
  • style – Scoped styles — changes inside won’t affect elements outside (still experimental).
  • strict – All of the above (size, layout, paint, and style containment).
  • content – Same as layout, paint, and style but without size containment.

When should you use containment?

Containment works best when applied to self-contained components — things like widgets, cards, or sidebars that don’t visually or layout-wise depend on other parts of the page.

For example:

  • A dynamic widget in a sidebar
  • A card component in a grid layout
  • A comment section that loads independently

In these cases, wrapping them with contain: content or contain: layout paint can help the browser skip unnecessary work during rendering.

Keep in mind:

  • Containment only helps if the browser actually uses the hints — not all browsers fully support every option yet.
  • Don’t overuse it. Applying contain to everything can backfire because it creates extra layers and memory overhead.

How does containment improve performance?

When you update part of a webpage — say, changing some text or toggling a class — the browser usually has to check:

  • How that change affects layout (reflow)
  • Which parts need to be repainted
  • Whether those changes affect other elements

With containment, the browser knows:

  • It doesn’t have to look outside the contained box for layout or painting
  • It can isolate the rendering process just to that element

So instead of scanning the entire page after a small change, it only needs to re-check that one contained block.

Think of it like sealing off a room before doing messy construction — nothing spills out, and cleanup is easier.


Some practical tips for using contain

Here are a few ways to get started with containment:

  • Use contain: content on cards or list items that load independently.
  • Apply contain: layout paint to modules that update dynamically (like live feeds or counters).
  • Avoid applying it to elements that rely heavily on external layout context, such as flex/grid children.

Also keep in mind:

  • contain: strict is powerful but aggressive — test carefully.
  • Browser support is decent for layout and paint containment, but size and style are less widely supported.
  • Performance gains are more noticeable on large, complex pages.

Basically, CSS containment gives developers a tool to help the browser optimize rendering. It's not always needed, but when used right, it can make your site feel faster and smoother.

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