Efficiently rendering a website relies on the User Agent being able to detect what parts of the page are being displayed, which parts might affect the currently-displayed section, and what can be ignored.
There are various heuristics that can be used to guess when a given sub-tree is independent of the rest of the page in some manner, but they’re fragile, so innocuous changes to a page may inadvertently make it flunk the heuristics and fall into a slow mode. There are also many things that would be good to isolate which are difficult or impossible to detect in a heuristic manner.
To alleviate these problems and allow strong, predictable isolation of a subtree from the rest of the page, this specification defines a contain property.
2. Strong Containment: the contain property
|Value:||none | strict | content | [ size || layout || style || paint ]|
|Applies to:||all elements|
|Computed value:||specified value|
|Canonical order:||per grammar|
The contain property allows an author to indicate that an element and its contents are, as much as possible, independent of the rest of the document tree. This allows user agents to utilize much stronger optimizations when rendering a page using contain properly, and allows authors to be confident that their page won’t accidentally fall into a slow code path due to an innocuous change.
- This value indicates that the property has no effect. The element renders as normal, with no containment effects applied.
- This value turns on all forms of containment for the element. In other words, it behaves the same as contain: size layout style paint;, so that its contents are guaranteed to have no effect on the rest of the page outside the element’s bounds.
This value turns on all forms of containment except size containment for the element.
In other words, it behaves the same as contain: layout style paint;.
Note: contain: content is reasonably "safe" to apply widely; its effects are fairly minor in practice, and most content won’t run afoul of its restrictions. However, because it doesn’t apply size containment, the element can still respond to the size of its contents, which can cause layout-invalidation to percolate further up the tree than desired. Use contain: strict when possible, to gain as much containment as you can.
- The value turns on size containment for the element. This ensures that the containing element can be laid out without needing to examine its descendants.
- This value turns on layout containment for the element. This ensures that the containing element is totally opaque for layout purposes; nothing outside can affect its internal layout, and vice versa.
- This value turns on style containment for the element. This ensures that, for properties which can have effects on more than just an element and its descendants, those effects don’t escape the containing element.
- This value turns on paint containment for the element. This ensures that the descendants of the containing element don’t display outside its bounds, so if an element is off-screen or otherwise not visible, its descendants are also guaranteed to be not visible.
For example, assume a micropost social network had markup something like this:
<body> <aside class='sidebar'>...</aside> <article class='messages'> <section class='message'> Lol, check out this dog: images.example.com/jsK3jkl </section> <section class='message'> I had a ham sandwich today. #goodtimes </section> <section class='message'> I have political opinions that you need to hear! </section> … </article> </body>
There are probably a lot of messages displayed on the site, but each is independent and won’t affect anything else on the site. As such, each can be marked with contain: content to communicate this to the user agent, so it can optimize the page and skip a lot of computation for messages that are off-screen. If the size of each message is known ahead of time, contain: strict can be applied to communicate further restrictions.
3. Types of Containment
There are several varieties of containment that an element can be subject to, restricting the effects that its descendants can have on the rest of the page in various ways. Containment enables much more powerful optimizations by user agents, and helps authors compose their page out of functional units, as it limits how widely an effect a given change can have on a document.
3.1. Size Containment
Giving an element size containment has the following effect:
When laying out the containing element, it must be treated as having no contents.
After layout of the element is complete, its contents must then be laid out into the containing element’s resolved size.
By itself, size containment does not offer much optimization opportunity. Its primary benefit on its own is that tools which want to lay out the containing element’s contents based on the containing element’s size (such as a JS library implementing the "container query" concept) can do so without fear of "infinite loops", where having a child’s size respond to the size of the containing element causes the containing element’s size to change as well, possibly triggering further changes in how the child sizes itself and possibly thus more changes to the containing element’s size, ad infinitum.
When paired with layout containment, though, possible optimizations that can be enabled include (but are not limited to):
When the style or contents of a descendant of the containing element is changed, calculating what part of the DOM tree is "dirtied" and might need to be re-laid out can stop at the containing element.
When laying out the page, if the containing element is off-screen or obscured, the layout of its contents can be delayed or done at a lower priority.
3.2. Layout Containment
Giving an element layout containment has the following effects:
The element must be a formatting context.
This needs to specify how it becomes a formatting context (and which type it becomes).
If a fragmentation context participates in layout containment, the first element with layout containment affecting the fragmentation context must “trap” the remainder of the fragmented flow. Fragmentation must not continue past the layout containment boundary, and the last fragmentation container within the first layout containment boundary is treated as if it is the last fragmentation container in its fragmentation context.
If subsequent fragmentation containers in the fragmentation context are only generated when more content remains in the fragmented flow, then they are not generated. If they would exist regardless, they remain part of the fragmentation context, but do not receive any content from the fragmented flow.
If the contents of the element overflow the element, they must not project any "geometry" beyond the element’s normal bounds. In particular, any overflow must not be capable of causing the element’s ancestors to overflow; as far as the ancestors are concerned, the element is perfectly confined to its normal width×height bounds.
Possible optimizations that can be enabled by layout containment include (but are not limited to):
When laying out the page, the contents of separate containing elements can be laid out in parallel, as they’re guaranteed not to affect each other.
When laying out the page, if the containing element is off-screen or obscured and the layout of the visible parts of the screen do not depend on the size of the containing element (for example, if the containing element is near the end of a block container, and you’re viewing the beginning of the block container), the layout of the containing elements' contents can be delayed or done at a lower priority.
(When paired with size containment, this optimization can be applied more liberally.)
3.3. Style Containment
Giving an element style containment has the following effects:
The following properties must have no effect on descendants of the element:
The counter-increment, counter-set, flow-from, flow-into, and content (for the purpose of open-quote/etc values) properties must be scoped to the element’s sub-tree.
A scoped property has its effects scoped to a particular element or subtree. It must act as if the scoping element was the root of the document for the purpose of evaluating the property’s effects: any uses of the property outside the scoping element must have no effect on the uses of the property on or in the scoping element, and vice versa. If scoped to a sub-tree, it’s the same, except the scoping element itself is counted as "outside" the tree, like the rest of the document.
For example, if counter-increment is scoped to an element, the first use of it within the subtree acts as if the named counter were set to 0 at the scoping element, regardless of whether the counter had been used outside the scoping element. Any increments made within the subtree have no effect on counters of the same name outside the scoping element.
Possible optimizations that can be enabled by style containment include (but are not limited to):
Whenever a property is changed on a descendant of the containing element, calculating what part of the DOM tree is "dirtied" and might need to have its style recalculated can stop at the containing element.
3.4. Paint Containment
Giving an element paint containment has the following effects:
If the computed value of overflow-x or overflow-y would otherwise have been visible, it must instead compute to clip. [CSS-OVERFLOW-3] This means that regardless of the specified value of overflow, the contents of the element is clipped to the element’s content box, including both the paint of the descendants and their geometry
The element must act as a containing block for absolutely positioned and fixed positioned descendants.
The element must be a stacking context.
The element must be a formatting context.
Possible optimizations that can be enabled by paint containment include (but are not limited to):
If the containing element is off-screen or obscured, the UA can directly skip trying to paint its contents, as they’re guaranteed to be off-screen/obscured as well.
If the containing element’s overflow has been changed to clip (because it was originally visible), the UA can reserve "canvas" space for the element exactly the element’s size. (In similar, scrollable, situations, like overflow: hidden, it’s possible to scroll to the currently-clipped content, so UAs often predictively overpaint somewhat so there’s something to see as soon as the scroll happens, rather than a frame later.)
Because they are guaranteed to be stacking contexts, scrolling elements can be painted into a single GPU layer.
4. Privacy and Security Considerations
This specification introduces no new privacy or security considerations.