The essentials

Quick reference

One focused task per row. Jump to the related section for complete, working examples.

UseSyntaxExamples
Name an anchor.trigger { anchor-name: --menu-trigger; }View examples
Select the default anchor.menu { position-anchor: --menu-trigger; }View examples
Enable anchored positioning.menu { position: fixed; }View examples
Place below.menu { position-area: block-end span-inline-end; }View examples
Place above.tooltip { position-area: block-start center; }View examples
Read an anchor edge.callout { inset-block-start: anchor(--target block-end, 0px); }View examples
Center on an anchor.callout { inset-inline-start: anchor(--target center); translate: -50% 0; }View examples
Match anchor width.menu { inline-size: anchor-size(inline, 12rem); }View examples
Bound popup size.menu { max-inline-size: min(24rem, calc(100vi - 2rem)); }View examples
Flip across block axis.menu { position-try-fallbacks: flip-block; }View examples
Declare fallback order.menu { position-try-fallbacks: flip-block, flip-inline, --compact; }View examples
Prefer available height.menu { position-try-order: most-height; }View examples
Define a custom option@position-try --compact { position-area: block-end; max-block-size: 12rem; }View examples
Hide invalid placement.tooltip { position-visibility: anchors-visible no-overflow; }View examples
Scope repeated names.widget { anchor-scope: --trigger; }View examples
Guard enhancement@supports (anchor-name: --a) { .enhanced { position-area: block-end; }; }View examples

Anchor positioning lets an absolutely or fixed-positioned box derive its placement and size from another element without measuring coordinates in JavaScript. Treat it as progressive enhancement: keep the trigger and popup semantically connected, provide a usable conventional-positioning fallback, choose logical directions for international layouts, and test clipping, zoom, scrolling, and narrow viewports. The syntax is still evolving, so verify the feature set in every browser you support.

Step by step

Detailed examples

01

Connect the positioned box to a named anchor

Give the source element a dashed anchor name, make the dependent box absolute or fixed, and select that anchor with position-anchor. Names are tree-scoped rather than global IDs, but repeated components should still limit name visibility with anchor-scope. Prefer fixed for viewport-layer popovers and absolute when the containing block is deliberately local.

Wire a menu to its trigger
.account-button {
  anchor-name: --account-button;
}

.account-menu {
  position: fixed;
  position-anchor: --account-button;
  margin: 0.5rem;
  z-index: 10;
}
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02

Choose a logical region around the anchor

position-area turns the containing block and anchor edges into a three-by-three positioning grid. Logical block and inline terms adapt to writing modes; span-* values let a popup occupy more than one cell. Normal inset and self-alignment declarations then operate within the selected region.

Place a tooltip above and a menu below
.tooltip {
  position: fixed;
  position-anchor: --help;
  position-area: block-start center;
  justify-self: anchor-center;
  margin-block-end: 0.4rem;
}

.menu {
  position: fixed;
  position-anchor: --actions;
  position-area: block-end span-inline-end;
  margin-block-start: 0.5rem;
}
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03

Read precise anchor edges with anchor()

Use anchor() in inset properties when a position-area cell is not precise enough or when different edges depend on different anchors. A named reference is optional when position-anchor establishes the default. Include a length-percentage fallback where unresolved references must degrade predictably, and remember that opposing insets can stretch the positioned box.

Align a callout to explicit edges
.target { anchor-name: --target; }

.callout {
  position: fixed;
  inset-block-start: calc(anchor(--target block-end, 0px) + 0.5rem);
  inset-inline-start: anchor(--target center, 50%);
  translate: -50% 0;
}
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04

Derive dimensions without sacrificing viewport safety

anchor-size() can feed sizing, inset, and margin properties on an absolutely positioned box. Its physical and logical size keywords may use a named anchor or the default anchor. Always combine matching with content and viewport constraints so long labels, large text, and narrow screens cannot make the overlay unusable.

Match a trigger while preserving a safe maximum
.select-menu {
  position: fixed;
  position-anchor: --select-trigger;
  inline-size: anchor-size(inline, 12rem);
  min-inline-size: min(12rem, calc(100vi - 2rem));
  max-inline-size: min(24rem, calc(100vi - 2rem));
  max-block-size: min(20rem, calc(100vb - 2rem));
  overflow: auto;
}
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05

Try alternate positions when space runs out

position-try-fallbacks supplies ordered overflow alternatives. Built-in flip-block, flip-inline, and flip-start tactics transform the base positioning; area values and named @position-try rules provide explicit choices. position-try-order can prefer the option with the most usable dimension, but it does not replace sensible maximum sizing and scrolling.

Flip a menu and prefer the taller side
.menu {
  position-area: block-end span-inline-end;
  position-try-fallbacks: flip-block, flip-inline, --compact;
  position-try-order: most-height;
  max-block-size: 18rem;
  overflow: auto;
}
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06

Define constrained custom position options

An @position-try rule may contain only inset, margin, sizing, self-alignment, position-anchor, and position-area declarations. It cannot restyle the popup arbitrarily, and !important makes the affected declaration invalid. Keep visual presentation in the ordinary rule and use the custom option strictly to describe alternate geometry.

Create a compact scrollable fallback
@position-try --compact {
  position-area: block-end;
  inline-size: min(18rem, calc(100vi - 2rem));
  max-block-size: 12rem;
  align-self: start;
}

.menu {
  position-try-fallbacks: flip-block, --compact;
}
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07

Scope components and ship a robust fallback

anchor-scope prevents identical names in repeated widgets from resolving outside their intended subtree. position-visibility can suppress geometry that is invalid, hidden, or overflowing, but hidden content still needs correct interaction state. Put a conventional fixed or absolute fallback first, then override inside @supports; CSS feature queries test parsing, not the quality of a specific browser implementation.

Progressively enhance a repeated tooltip
.help { position: relative; anchor-scope: --help-trigger; }
.help__tip { position: absolute; inset: auto 0 100% auto; }

@supports (anchor-name: --a) and (position-area: block-start) {
  .help__button { anchor-name: --help-trigger; }
  .help__tip {
    position: fixed;
    position-anchor: --help-trigger;
    position-area: block-start center;
    position-visibility: anchors-visible no-overflow;
    inset: auto;
  }
}
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Local code tester

Anchor a menu with a conventional fallback

The menu works with ordinary positioning and upgrades to anchor positioning when the browser supports it.

Runs in your browser
Preview

Keep exploring

More CSS cheat sheets

Sources and further reading

References

Authoritative documentation used to verify and expand this cheat sheet.

  1. World Wide Web ConsortiumCSS Anchor Positioning Module Level 1w3.org
  2. World Wide Web ConsortiumCSS Positioned Layout Module Level 3w3.org
  3. World Wide Web ConsortiumCSS Conditional Rules Module Level 3w3.org
  4. World Wide Web ConsortiumCSS Writing Modes Level 4w3.org

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