Use CSS contain to tell the browser which parts of an element’s rendering can be handled independently from the rest of the page. For example, .card { contain: layout paint; } isolates layout and clips paint overflow, but it also changes positioning and stacking behavior. Choose the smallest containment value that fits the component, then check overlays and overflow.
What CSS containment does
The contain property declares that an element and its contents are, as much as possible, independent from the rest of the document tree. That can limit how far certain layout, style, paint, or size calculations need to reach when content changes. It is a component-behavior declaration, not a universal performance switch; measure your actual page before claiming a speed improvement.
Containment is most useful when a component has a clear boundary and its internal rendering can be isolated. It may be a poor fit when the component relies on children to determine its dimensions or deliberately lets content draw outside its box.
Choose the containment value
| Value | What it contains | Practical effect |
|---|---|---|
none |
Nothing | Default behavior; no containment is applied. |
layout |
Layout | Isolates internal layout from the surrounding page. |
paint |
Paint | Clips descendants that paint outside the containing box. |
size |
Size in both inline and block directions | The element’s size is computed independently of its children. Do not combine with inline-size. |
inline-size |
Size in the inline direction | Inline size is computed independently of child content. Do not combine with size. |
style |
Relevant style effects, such as CSS counters | Does not scope ordinary CSS styles like Shadow DOM or @scope. |
content |
layout paint style |
A shorthand that does not include size containment. |
strict |
size layout paint style |
Applies the strongest listed combination; the element may size as though it had no children. |
You can combine values, for example contain: layout paint; or contain: inline-size layout;. Use only the types needed for the boundary you intend to create. The MDN reference documents contain and its values at MDN: contain CSS property.
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Apply containment without breaking the component
- Identify the component boundary. Choose a subtree whose layout, paint, or sizing behavior can be isolated without changing required interactions with the rest of the page.
- Start with the minimum value. If the goal is layout isolation, try
contain: layout;. Addpaintonly if clipping overflow is intended; add size containment only when the component has suitable dimensions independent of its children. - Provide dimensions when using size containment. With
sizeorstrict, the element’s dimensions no longer follow its contents in the usual way. Set appropriate dimensions or usecontain-intrinsic-sizewhen a useful fallback size is needed. - Test visual and positioned content. Check menus, tooltips, shadows, fixed and absolutely positioned descendants, and z-index relationships. Layout and paint containment, as well as
contentandstrict, create a containing block for positioned descendants, a stacking context, and a block formatting context. Paint containment clips overflow. - Profile the relevant interaction. Compare the page with and without containment under the updates that matter. A container whose own dimensions depend on its children may still need its size recalculated when those contents change.
Example: isolate layout while retaining overflow
For a component that needs layout isolation but has a tooltip or shadow extending outside its box, test layout containment without paint containment:
.card {
contain: layout;
}This still changes positioned descendants and stacking behavior, so verify them in context.
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Example: intentionally clip paint
If content should never render beyond the component boundary, include paint containment:
.thumbnail {
contain: layout paint;
}Because paint clips overflow, this can cut off menus, shadows, or other content that extends beyond the element. Move such content outside the contained element or choose a value without paint containment if it must remain visible.
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Containment, content visibility, and intrinsic size
These related properties address different needs. contain isolates selected rendering behavior; content-visibility controls whether an element’s contents render and can let the browser defer rendering work until needed; contain-intrinsic-size supplies a size used for layout when size containment applies. For the latter property, see MDN: contain-intrinsic-size. MDN’s guide to the broader topic is Using CSS containment.
Browser support
MDN marks contain as Baseline and widely available, with cross-browser availability since March 2022. Its reference follows CSS Containment Module Level 2 and notes that not every part may be implemented in every browser. Check the specific value against your project’s browser targets rather than assuming every syntax detail is supported everywhere.
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Common problems and fixes
- A tooltip, menu, or shadow is cut off. The element or an ancestor may use
paint,content, orstrict. Remove paint containment where overflow must remain visible, or place the overlay outside the clipped subtree. - A positioned child appears in an unexpected place.
layout,paint,content, andstrictestablish a containing block for absolutely and fixed-positioned descendants. Review the child’s positioning relative to the new containing block. - A child’s z-index no longer competes with elements elsewhere. The same containment values create a stacking context. Adjust the stacking structure or place the element that must layer globally outside that context.
- The box collapses or no longer fits its contents. You may have applied
sizeorstrict. Give the component appropriate dimensions or provide a fallback withcontain-intrinsic-size. - The declaration does not scope ordinary CSS rules.
stylecontainment scopes relevant effects such as counters; it is not a replacement for Shadow DOM or@scope. - There is no visible performance improvement. Containment does not make all page updates free. Confirm that the targeted work is affected and profile the actual interaction; avoid adding containment where the component’s behavior conflicts with it.
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