CSS Containment Specification

The aim of the CSS Containment specification is to improve performance of web pages by allowing developers to isolate a subtree of the page from the rest of the page. If the browser knows that a part of the page is independent, rendering can be optimized and performance improved. The specification defines a single CSS property contain

. This document describes the basic aims of the specification.

Basic example

Many webpages contain a number of sections which are independent of each other. For example a listing of article headlines and content, as in the mark-up below.

<h1>My blog</h1>
<article>
  <h2>Heading of a nice article</h2>
  <p>Content here.</p>
</article>
<article>
  <h2>Another heading of another article</h2>
  <p>More content here.</p>
</article>

Each article has the contain property with a value of content applied in the CSS.

article {
  contain: content;
}

Each article is independent of the other articles on the page, and so they have been given contain: content in order to indicate to the browser that this is the case. The browser can then use this information to make decisions about how to render the content. For example, it might not render articles that are outside the viewable area.

If we give each <article> the contain property with a value of content , when new elements are inserted the browser understands it does not need to relayout or repaint any area outside of the containing element's subtree, although if the <article> is styled such that its size depends on its contents (e.g. with height: auto ), then the browser may need to account for its size changing).

We have told it by way of the contain property that each article is independent.

The content value is shorthand for contain: layout paint . It tells the browser that the internal layout of the element is totally separate from the rest of the page, and that everything about the element is painted inside its bounds. Nothing can visibly overflow.

This information is something that is usually known, and in fact quite obvious, to the web developer creating the page. However browsers cannot guess at your intent and cannot assume that an article will be entirely self-contained. Therefore this property gives you a nice way to explain to the browser this fact, and allow it to make performance optimizations based on that knowledge.

Key concepts and terminology

This specification defines only one property, the contain property. The values for this property indicate the type of containment that you want to apply to that element.

Layout containment

article {
  contain: layout;
}

Layout is normally scoped to the entire document, which means that if you move one element the entire document needs to be treated as if things could have moved anywhere. By using contain: layout you can tell the browser it only needs to check this element — everything inside the element is scoped to that element and does not affect the rest of the page, and the containing box establishes an independent formatting context.

In addition:

• float layout will be performed independently.
• Margins won't collapse across a layout containment boundary
• The layout container will be a containing block for absolute / fixed position descendants.
• The containing box creates a stacking context, therefore  z-index can be used.

Paint containment

article {
  contain: paint;
}

Paint containment essentially clips the box to the padding edge of the principal box. There can be no visible overflow. The same things are true for paint containment as layout containment (see above).

Another advantage is that if the containing box is offscreen, the browser does not need to paint its contained elements — these must also be offscreen as they are contained completely by that box.

Size containment

article {
  contain: size;
}

Size containment does not offer much in the way of performance optimizations when used on its own. However, it means that the size of the element's children cannot affect the size of the element itself — its size is computed as if it had no children.

If you turn on contain: size you need to also specify the size of the element you have applied this to. It will end up being zero-sized in most cases, if you don't manually give it a size.

Style containment

article {
  contain: style;
}

Despite the name, style containment does not provide scoped styles such as you would get with theShadow DOM. The main use case is to prevent situations where aCSS Counter could be changed in an element, which could then affect the rest of the tree. 

Using contain: style would ensure that the  counter-increment and  counter-set properties created new counters scoped to that subtree only.

Note: style containment is "at-risk" in the spec and may not be supported everywhere (it's not currently supported in Firefox).

Special values

There are two special values of contain:

content
strict

We encountered the first in the example above. Using contain: content turns on layout and paint containment. The specification describes this value as being "reasonably safe to apply widely". It does not apply size containment, so you would not be at risk of a box ending up zero-sized due to a reliance on the size of its children.

To gain as much containment as possible use contain: strict , which behaves the same as contain: size layout paint , or perhaps the following to also add style containment in browsers that support it:

contain: strict;
contain: strict style;

Reference

CSS Properties

• contain

External resources

• An Introduction to CSS Containment