CSS Anchor Positioning

1. Introduction

CSS absolute positioning allows authors to place boxes anywhere on the page, without regard to the layout of other boxes besides their containing block. This flexibility can be very useful, but also very limiting—​often you want to position relative to some other box. Anchor positioning (via the position-anchor and position-area properties and/or the anchor functions anchor() and anchor-size()) allows authors to achieve this, “anchoring” an absolutely positioned box to one or more other boxes on the page (its anchor references, while also allowing them to try several possible positions to find the “best” one that avoids overlap/overflow.

.anchor {
  anchor-name: --tooltip;
}
.tooltip {
  /* Fixpos means we don’t need to worry about
     containing block relationships;
     the tooltip can live anywhere in the DOM. */
  position: fixed;

  /* All the anchoring behavior will default to
     referring to the --tooltip anchor. */
  position-anchor: --tooltip;

  /* Align the tooltip’s bottom to the top of the anchor;
     this also defaults to horizontally center-aligning
     the tooltip and the anchor (in horizontal writing modes). */
  position-area: block-start;

  /* Automatically swap if this overflows the window
     so the tooltip’s top aligns to the anchor’s bottom
     instead. */
  position-try: flip-block;

  /* Prevent getting too wide */
  max-inline-size: 20em;
}

1.1. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

Like most operations in CSS besides selector matching, features in this specification operate over the flattened element tree.

2. Determining the Anchor

2.1. Creating an Anchor: the anchor-name property

The anchor-name property declares that an element is an anchor element, whose principal box is an anchor box, and gives it a list of anchor names to be targeted by. Values are defined as follows:

none

The property has no effect.

<dashed-ident>#

If the element generates a principal box, the element is an anchor element, with a list of anchor names as specified. Each anchor name is a tree-scoped name.

Otherwise, the property has no effect.

Anchor names do not need to be unique. Not all elements are capable of being the target anchor element of a given box. Thus a name can be reused in multiple places if the usages are scoped appropriately.

Note: If multiple elements share an anchor name and are all visible to a given positioned box, the target anchor element will be the last one in DOM order. The anchor-scope property can be used to further limit what names are visible to a given referencing box.

Anchor names are not scoped by containment by default; even if an element has style or layout containment (or any similar sort of containment), the anchor names of its descendants are visible to elements elsewhere in the page.

Note: While an element is in the skipped contents of another element (due to content-visibility: hidden, for instance), it’s not an acceptable anchor element, effectively acting as if it had no names.

2.1.1. Implicit Anchor Elements

Some specifications can define that, in certain circumstances, a particular element is an implicit anchor element for another element.

TODO: Fill in an example new popover-related details (once that finally lands in the HTML spec).

Implicit anchor elements can be referenced with the auto keyword in position-anchor, or by omitting the anchor reference in anchor functions.

Pseudo-elements have the same implicit anchor element as their originating element, unless otherwise specified.

Note: Without this, these pseudo-elements, which are often inaccessible by other specifications, cannot be positioned with implicit anchor elements.

2.1.2. The Anchor’s Position

Several features of this specification refer to the position and size of an anchor box. The anchor box's position and size is determined after layout, and for these purposes includes position-based adjustments (such as position: relative or position: sticky).

Post-layout effects, such as transform, do not affect the anchor box’s position.

Note: Allowing an anchor to opt into including the effects of transform or similar properties might be allowed in the future.

2.2. Scoping Anchor Names: the anchor-scope property

This property scopes the specified anchor names, and lookups for these anchor names, to this element’s subtree. See § 2 Determining the Anchor.

Values have the following meanings:

none

No changes in anchor name scope.

all

Specifies that all anchor names defined by this element or its descendants—​whose scope is not already limited by a descendant using anchor-scope—​to be in scope only for this element’s descendants; and limits descendants to only match anchor names to anchor elements within this subtree.

<dashed-ident>

Specifies that a matching anchor name defined by this element or its descendants—​whose scope is not already limited by a descendant using anchor-scope—​to be in scope only for this element’s descendants; and limits descendants to only match these anchor names to anchor elements within this subtree.

This property has no effect on implicit anchor elements.

li {
  anchor-name: --list-item;
  anchor-scope: --list-item;
}
li .positioned {
  position: absolute;
  position-anchor: --list-item;
  position-area: inline-start;
}

2.3. Finding an Anchor

Several things in this specification find a target anchor element, given an anchor specifier, which is either a <dashed-ident> (and a tree-scoped reference) that should match an anchor-name value elsewhere on the page, or the keyword auto, or nothing (a missing specifier).

Note: The general rule captured by these conditions is that an element can only be a positioned box’s target anchor element if its own box is fully laid out before the positioned box that wants to reference it is laid out. CSS’s layout rules provide some useful guarantees about this, depending on the anchor and positioned box’s relationship with each other and their containing blocks. The list of conditions below exactly rephrases the stacking context rules into just what’s relevant for this purpose, ensuring there is no possibly circularity in anchor positioning.

2.4. Default Anchors: the position-anchor property

The position-anchor property specifies the default anchor element, which is used by position-area, position-try, and (by default) all anchor functions applied to this element. position-anchor is a reset-only sub-property of position.

auto

Use the implicit anchor element if it exists; otherwise the box has no default anchor element.

<anchor-name>

The target anchor element selected by the specified <anchor-name> is the box’s default anchor element.

The principal box of the default anchor element is the box’s default anchor box.

.anchored {
  position: absolute;
  top: calc(.5em + anchor(outside));
  /* Since no anchor name was specified,
     this automatically refers to the
     default anchor box. */
}

.foo.anchored {
  position-anchor: --foo;
}
.bar.anchored {
  position-anchor: --bar;
}

2.5. Anchor Relevance

When determining whether an element el is relevant to the user, if a descendant of el is a target anchor element for a positioned box (which itself is not skipped and whose containing block is not el or a descendant of el), then el must be considered relevant to the user.

Note: This means that, for example, an anchor in a content-visibility: auto subtree will prevent its subtree from skipping its contents as long as the positioned box relying on it is also not skipped. (Unless the anchor and the positioned box are both under the same content-visibility: auto element; they can’t cyclicly keep each other visible.)

3. Anchor-Based Positioning

An absolutely positioned box can position itself relative to one or more anchor boxes on the page.

The position-area property offers a convenient grid-based concept for positioning relative to the default anchor box; for more complex positioning or positioning relative to multiple boxes, the anchor() function can be used in the inset properties to explicitly refer to edges of an anchor box.

3.1. The position-area Property

Most common use-cases of anchor positioning are only concerned with the edges of the positioned box’s containing block and the edges of the default anchor box. These lines can be thought of as defining a 3×3 grid; position-area lets you easily specify what area of this position-area grid to lay out the positioned box in. Its syntax is:

<position-area> = [
  [ left | center | right | span-left | span-right
  | x-start | x-end | span-x-start | span-x-end
  | x-self-start | x-self-end | span-x-self-start | span-x-self-end
  | span-all ]
  ||
  [ top | center | bottom | span-top | span-bottom
  | y-start | y-end | span-y-start | span-y-end
  | y-self-start | y-self-end | span-y-self-start | span-y-self-end
  | span-all ]
|
  [ block-start | center | block-end | span-block-start | span-block-end | span-all ]
  ||
  [ inline-start | center | inline-end | span-inline-start | span-inline-end
  | span-all ]
|
  [ self-block-start | center | self-block-end | span-self-block-start
  | span-self-block-end | span-all ]
  ||
  [ self-inline-start | center | self-inline-end | span-self-inline-start
  | span-self-inline-end | span-all ]
|
  [ start | center | end | span-start | span-end | span-all ]{1,2}
|
  [ self-start | center | self-end | span-self-start | span-self-end | span-all ]{1,2}
]

none

The property has no effect.

<position-area>

If the box does not have a default anchor box, or is not an absolutely positioned box, this value has no effect.

Otherwise, it has the following effects:

• The property selects a region of the position-area grid, and makes that the box’s containing block. See § 3.1.1 Resolving <position-area>s for details.

  Note: This means that the inset properties specify offsets from the position-area, and some property values, like max-height: 100%, will be relative to the position-area as well.

• The normal value for the self-alignment properties behaves as either start, end, or anchor-center, depending on the positioning of the region, to give a good default alignment for the positioned box. Again, see § 3.1.1 Resolving <position-area>s for details.

• Any auto inset properties resolve to 0.

• The box is not considered to be in a legacy alignment mode in either axis.

3.1.1. Resolving <position-area>s

The position-area grid is a 3×3 grid, composed of four grid lines in each axis. In order (and using the writing mode of the containing block):

• the start edge of the box’s pre-modification containing block, or the start edge of the default anchor box if that is more start-ward

• the start edge of the default anchor box

• the end edge of the default anchor box

• the end edge of the box’s pre-modification containing block, or the end edge of the default anchor box if that is more end-ward.

A <position-area> selects a region of this grid by specifying the rows and columns the region occupies, with each of the two keywords specifying one of them:

start, end, self-start, self-end

top, bottom, left, right

y-start, y-end, y-self-start, y-self-end

x-start, x-end, x-self-start, x-self-end

block-start, block-end, block-self-start, block-self-end

inline-start, inline-end, inline-self-start, inline-self-end

center

The single corresponding row or column, depending on which axis this keyword is specifying.

Like in anchor(), the plain logical keywords (start, end, etc) refer to the writing mode of the box’s containing block. The x-start/etc determine their direction in the same way, but in the specified physical axis.

The self-* logical keywords (self-start, x-self-end, etc) are identical, but refer to the box’s own writing mode.

span-start, span-end

span-top, span-bottom

span-y-start, span-y-end

span-x-start, span-x-end

span-block-start, span-block-end

span-inline-start, span-inline-end

Two rows or columns, depending on which axis this keyword is specifying: the center row/column, and the row/column corresponding to the other half of the keyword as per the single-track keywords.

(For example, span-top spans the first two rows—​the center row and the top row.)

span-all

All three rows or columns, depending on which axis this keyword is specifying.

Some keywords are ambiguous about what axis they refer to: center, span-all, and the start/etc keywords that don’t specify the block or inline axis explicitly. If the other keyword is unambiguous about its axis, then the ambiguous keyword is referring to the opposite axis. (For example, in block-start center, the center keyword is referring to the inline axis.) If both keywords are ambiguous, however, then the first refers to the block axis of the box’s containing block, and the second to the inline axis. (For example, span-all start is equivalent to span-all inline-start.)

If only a single keyword is given, it behaves as if the second keyword is span-all if the given keyword is unambigous about its axis; otherwise, it behaves as if the given keyword was repeated. (For example, top is equivalent to top span-all, but center is equivalent to center center.)

The <position-area> also implies a default self-alignment, which will be used if the self-alignment property on the box is normal:

• If the only the center region in an axis is selected, or all three regions are selected, the default alignment in that axis is anchor-center.

• Otherwise, the default alignment in that axis is toward the non-specified side region: if it’s specifying the "start" region of its axis, the default alignment in that axis is end; etc.

For example, assuming an English-equivalent writing mode (horizontal-tb, ltr), then the value span-x-start top resolves to the start region of the vertical axis, and the start and center regions of the horizontal axis, so the default alignments will be align-self: end (making the box’s bottom margin edge flush with the bottom of the top region) and justify-self: end (making the box’s end-side margin edge flush with the end side of the span-start region).

Note: When the default anchor box is partially or completely outside of the pre-modified containing block, some of the position-area grid’s rows or columns can be zero-sized.

3.2. The anchor() Function

An absolutely positioned box can use the anchor() function as a value in its inset properties to refer to the position of one or more anchor boxes. The anchor() function resolves to a <length>. It is only valid to be used in the inset properties.

<anchor()> = anchor( <anchor-name>? && <anchor-side>, <length-percentage>? )
<anchor-name> = <dashed-ident>
<anchor-side> = inside | outside
       | top | left | right | bottom
       | start | end | self-start | self-end
       | <percentage> | center

The anchor() function has three arguments:

• the <anchor-name> value specifies how to find the anchor element it will be drawing positioning information from. Its possible values are:

  <dashed-ident>

  Specifies the anchor name it will look for. This name is a tree-scoped reference.

  omitted

  Selects the default anchor element defined for the box, if possible.

  See target anchor element for details.

• the <anchor-side> value refers to the position of the corresponding side of the target anchor element. Its possible values are:

  inside

  outside

  Resolves to one of the anchor box’s sides, depending on which inset property it’s used in. inside refers to the same side as the inset property (attaching the positioned box to the "inside" of the anchor box), while outside refers to the opposite.

  top

  right

  bottom

  left

  Refers to the specified side of the anchor box.

  Note: These are only usable in the inset properties in the matching axis. For example, left is usable in left, right, or the logical inset properties that refer to the horizontal axis.

  start

  end

  self-start

  self-end

  Refers to one of the sides of the anchor box in the same axis as the inset property it’s used in, by resolving the keyword against the writing mode of either the positioned box (for self-start and self-end) or the positioned box’s containing block (for start and end).

  <percentage>

  center

  Refers to a position a corresponding percentage between the start and end sides, with 0% being equivalent to start and 100% being equivalent to end.

  center is equivalent to 50%.

• the optional <length-percentage> final argument is a fallback value, specifying what the function should resolve to if it’s an invalid anchor function.

An anchor() function representing a valid anchor function resolves at computed value time (using style & layout interleaving) to the <length> that would align the edge of the positioned boxes' inset-modified containing block corresponding to the property the function appears in with the specified border edge of the target anchor element, assuming that all scroll containers between the target anchor element and the positioned box’s containing block are scrolled to their initial scroll position (but see § 3.4 Taking Scroll Into Account).

Note: This means that transitions or animations of a property using an anchor function will work "as expected" for all sorts of possible changes: the anchor box moving, anchor elements being added or removed from the document, the anchor-name property being changed on anchors, etc.

If the target anchor element is fragmented, the axis-aligned bounding rectangle of the fragments' border boxes is used instead.

.centered-message {
  position: fixed;
  max-width: max-content;
  justify-self: center;

  --center: anchor(--x 50%);
  --half-distance: min(
    abs(0% - var(--center)),
    abs(100% - var(--center))
  );
  left: calc(var(--center) - var(--half-distance));
  right: calc(var(--center) - var(--half-distance));
  bottom: anchor(--x top);
}

3.3. Centering on the Anchor: the anchor-center value

The self-alignment properties allow an absolutely positioned box to align itself within the inset-modified containing block. The existing values, plus carefully chosen inset properties, are usually enough for useful alignment, but a common case for anchored positioning—​centering over the anchor box—​requires careful and somewhat complex set-up to achieve.

The new anchor-center value makes this case extremely simple: if the positioned box has a default anchor box, then it is centered (insofar as possible) over the default anchor box in the relevant axis.

Additionally, any auto inset properties resolve to 0.

If the box is not absolutely positioned, or does not have a default anchor box, this value behaves as center and has no additional effect on how inset properties resolve.

Note: Similar to position-area, when using anchor-center, if the anchor is too close to the edge of the box’s original containing block, by default it will “shift” from being purely centered, in order to remain within the original containing block. See CSS Box Alignment 3 § 4.4 Overflow Alignment: the safe and unsafe keywords and scroll safety limits for more details.

3.4. Taking Scroll Into Account

For performance reasons, implementations usually perform scrolling on a separate scrolling/"compositing" thread, which has very limited capabilities (simple movement/transforms/etc., but no layout or similar expensive operations) and thus can be relied upon to respond to scrolling fast enough to be considered "instant" to human perception.

If scrolling just causes an anchor-positioned element to move, there is in theory no issue; the movement can be performed on the scrolling thread so the positioned element moves smoothly with the scrolling content. However, anchor positioning allows an element to make the positions of its own opposite edges depend on things in different scrolling contexts, which means scrolling could move just one edge and cause a size change, and thus perform layout. This can’t be performed on the scrolling thread!

To compensate for this, while still allowing as much freedom to anchor to various elements as possible, anchor positioning uses a combination of remembered scroll offsets and compensating for scroll.

The above allows a positioned element to respond to the scroll positions of its anchor references once, but if any of them are scrolled, the positioned element will no longer appear to be anchored to them (tho it will continue to respond to their non-scrolling movement). While this problem can’t be solved in general, we can respond to the scrolling of one anchor reference; specifically, the default anchor element:

3.5. Validity

An anchor() function is a valid anchor function only if all the following conditions are true:

• It’s being used in an inset property on an absolutely positioned box.

• If its <anchor-side> specifies a physical keyword, it’s being used in an inset property in that axis. (For example, left can only be used in left, right, or a logical inset property in the horizontal axis.)

• The result of determining the target anchor element is not nothing when given the querying element as the element it’s used on, and the anchor specifier as the <anchor-name> value specified in the function.

If any of these conditions are false, the anchor() function resolves to its specified fallback value. If no fallback value is specified, it makes the declaration referencing it invalid at computed-value time.

3.6. Conditional Hiding: the position-visibility property

There are some conditions in which it might not make sense to display an absolutely positioned box. This property allows such boxes to be made conditionally visible, depending on some commonly needed layout conditions.

always

This property has no effect. (The box is displayed without regard for its anchors or its overflowing status.)

anchors-valid

If any of the box’s required anchor references do not resolve to a target anchor element, the box is strongly hidden.

What is a required anchor reference? anchor() functions that don’t have a fallback value; the default anchor *sometimes*? Need more detail here.

Any anchors are missing, or all anchors are missing? I can see use-cases for either, potentially. Do we want to make a decision here, or make it controllable somehow?

anchors-visible

If the box has a default anchor box but that anchor box is clipped by intervening boxes, this box is also strongly hidden.

no-overflow

If the box overflows its inset-modified containing block even after applying position-try, then the box is strongly hidden.

A box that is strongly hidden acts as if it and all of its descendants were visibility: hidden, regardless of what their actual visibility value is.

3.7. Accessibility Implications

It’s important to remember that Anchor Positioning does not automatically establish any semantic relationship between a positioned box and any of its anchors, because it can be used in many different ways. Authors must not rely solely on a visual connection implied by the positioning to link elements together semantically; without additional help, the elements often have no meaningful DOM relationship, making them difficult or impossible to use in non-visual user agents, like screen readers.

Many features on the web platform, both existing and upcoming, allow establishing such connections explicitly, so that non-visual user agents can also benefit.

For example, the Popover API in HTML automatically links the invoker button to the popover element, including automatically adjusting tabbing order; it also establishes the invoker button as the implicit anchor element for the popover, making it easy to use Anchor Positioning as well.

In more general cases, ARIA features such as the aria-details or aria-describedby attributes on an anchor element can provide this information in a slightly more manual fashion; in concert with the role attribute on the positioned element, non-visual user agents can tell their users about the relationship between the elements and let them automatically navigate between them.

4. Anchor-Based Sizing

An absolutely positioned box can use the anchor-size() function in its sizing properties to refer to the size of one or more anchor boxes. The anchor-size() function resolves to a <length>. It is only valid to be used in the accepted @position-try properties.

4.1. The anchor-size() Function

anchor-size() = anchor-size( [ <anchor-name> || <anchor-size> ]? , <length-percentage>? )
<anchor-size> = width | height | block | inline | self-block | self-inline

The anchor-size() function is similar to anchor(), and takes the same arguments, save that the <anchor-side> keywords are replaced with <anchor-size>, referring to the distance between two opposing sides.

The physical <anchor-size> keywords (width and height) refer to the width and height, respectively, of the target anchor element. Unlike anchor(), there is no restriction on having to match axises; for example, width: anchor-size(--foo height); is valid.

The logical <anchor-size> keywords (block, inline, self-block, and self-inline) map to one of the physical keywords according to either the writing mode of the box (for self-block and self-inline) or the writing mode of the box’s containing block (for block and inline).

If the <anchor-size> keyword is omitted, it defaults to behaving as whatever keyword matches the axis of the property that anchor-size() is used in. (For example, width: anchor-size() is equivalent to width: anchor-size(width).)

An anchor-size() function representing a valid anchor-size function resolves at computed value time (via style & layout interleaving) to the <length> separating the relevant border edges (either left and right, or top and bottom, whichever is in the specified axis) of the target anchor element.

4.2. Validity

An anchor-size() function is a valid anchor-size function only if all the following conditions are true:

• It’s being used in a sizing property, an inset property, or a margin property on an absolutely positioned box. (These are the accepted @position-try properties that allow lengths.)

• There is a target anchor element for the box it’s used on, and the <anchor-name> value specified in the function.

If any of these conditions are false, the anchor-size() function resolves to its specified fallback value. If no fallback value is specified, it makes the declaration referencing it invalid at computed-value time.

5. Overflow Management

Anchor positioning, while powerful, can also be unpredictable. The anchor box might be anywhere on the page, so positioning a box in any particular fashion (such as above the anchor, or the right of the anchor) might result in the positioned box overflowing its containing block or being positioned partially off screen.

To ameliorate this, an absolutely positioned box can use the position-try-fallbacks property to refer to several variant sets of positioning/alignment properties (generated from the box’s existing styles, or specified in @position-try rules) that the UA can try if the box overflows its initial position. Each is applied to the box, one by one, and the first that doesn’t cause the box to overflow its containing block is taken as the winner.

position-try-order allows these options to additional be sorted by the available space they define, if it’s more important for the box to have as much space as possible rather than strictly follow some declared order.

5.1. Giving Fallback Options: the position-try-fallbacks property

This property provides a list of alternate positioning styles to try when the absolutely positioned box overflows its inset-modified containing block. This position options list is initially empty.

Each comma-separated entry in the list is a separate option: either the name of a @position-try block, or a <try-tactic> representing an automatic transformation of the box’s existing computed style.

Values have the following meanings:

none

The property has no effect; the box’s position options list is empty.

<dashed-ident>

If there is a @position-try rule with the given name, its associated position option is added to the position options list.

Otherwise, this value has no effect.

<try-tactic>

Automatically creates a position option from the box’s computed style, by swapping due to a try-tactic according to the specified keyword, and adding the constructed position option to the box’s position options list. and adds it to the position options list.

<try-tactic> = flip-block || flip-inline || flip-start

flip-block

swaps the values in the block axis (between, for example, margin-block-start and margin-block-end), essentially mirroring across an inline-axis line.

flip-inline

swaps the values in the inline axis, essentially mirroring across a block-axis line.

flip-start

swaps the values of the start properties with each other, and the end properties with each other (between, for example, margin-block-start and margin-inline-start), essentially mirroring across a diagonal drawn from the start-start corner to the end-end corner.

If multiple keywords are given, the transformations are composed in order to produce a single position option.

<dashed-ident> || <try-tactic>

Combines the effects of the previous two options: if there is a @position-try rule with the given name, applies its position option to the base style, then transforms it according to the specified <try-tactic> and adds the result to the box’s position options list.

Otherwise, does nothing.

<'position-area'>

Automatically creates a position option composed solely of a position-area property with the given value.

5.2. Determining Fallback Order: the position-try-order property

This property specifies the order in which the position options list will be tried.

<try-size> = most-width | most-height | most-block-size | most-inline-size

normal

Try the position options in the order specified by position-try-fallbacks.

most-width

most-height

most-block-size

most-inline-size

For each entry in the position options list, apply that position option to the box, and find the specified inset-modified containing block size that results from those styles. Stably sort the position options list according to this size, with the largest coming first.

.anchor { anchor-name: --foo; }
.list {
  position: fixed;
  position-anchor: --foo;
  position-area: block-end span-inline-end;
  align-self: start;
  position-try-fallbacks: --bottom-scrollable, flip-block, --top-scrollable;
  position-try-order: most-height;
}
@position-try --bottom-scrollable {
  align-self: stretch;
}
@position-try --top-scrollable {
  position-area: block-start span-inline-end;
  align-self: stretch;
}

5.3. The position-try Shorthand

This shorthand sets both position-try-fallbacks and position-try-order. If <'position-try-order'> is omitted, it’s set to the property’s initial value.

5.4. The @position-try Rule

The @position-try rule defines a position option with a given name, specifying one or more sets of positioning properties that can be applied to a box via position-try-fallbacks,

The syntax of the @position-try rule is:

@position-try <dashed-ident> {
  <declaration-list>
}

The <dashed-ident> specified in the prelude is the rule’s name. If multiple @position-try rules are declared with the same name, the last one in document order "wins".

The @position-try rule only accepts the following properties:

• inset properties

• margin properties

• sizing properties

• self-alignment properties

• position-anchor

• position-area

It is invalid to use !important on the properties in the <declaration-list>. Doing so causes the property it is used on to become invalid, but does not invalidate the @property-try rule as a whole.

All of the properties in a @position-try are applied to the box as part of the Position Fallback Origin, a new cascade origin that lies between the Author Origin and the Animation Origin.

Similar to the Animation Origin, use of the revert value acts as if the property was part of the Author Origin, so that it instead reverts back to the User Origin. (As with the Animation Origin, however, revert-layer has no special behavior and acts as specified.)

Note: The accepted @position-try properties are the smallest group of properties that affect just the size and position of the box itself, without otherwise changing its contents or styling. This significantly simplifies the implementation of position fallback while addressing the fundamental need to move an anchor-positioned box in response to available space. Since these rules override normal declarations in the Author Origin, this also limits the poor interactions of @position-try declarations with the normal cascading and inheritance of other properties. It is expected that a future extension to container queries will allow querying an element based on the position fallback it’s using, enabling the sort of conditional styling not allowed by this restricted list.

Note: If multiple boxes using different anchors want to use the same fallback positioning, just relative to their own anchor elements, omit the <anchor-name> in anchor() and specify each box’s anchor in position-anchor instead.

Note: The most common types of fallback positioning (putting the positioned box on one side of the anchor normally, but flipping to the opposite side if needed) can be done automatically with keywords in position-try-fallbacks, without using @position-try at all.

5.5. Applying Position Fallback

When a positioned box (shifted by its default scroll shift) overflows its inset-modified containing block, and has a non-empty position options list, it determines position fallback styles to attempt to find an option that avoids overflow.

These modified styles are applied to the element via interleaving, so they affect computed values (and can trigger transitions/etc) even though they depend on layout and used values.

During a full layout pass, once a box has determined its fallback styles (or determined it’s not using any), laying out later boxes cannot change this decision.

Implementations may choose to impose an implementation-defined limit on the length of position options lists, to limit the amount of excess layout work that may be required. This limit must be at least five.

#myPopover {
  position: fixed;
  top: anchor(--button bottom);
  left: anchor(--button left);
  position-try-fallbacks: flip-inline, flip-block, flip-block flip-inline;

  /* The popover is at least as wide as the button */
  min-width: anchor-size(--button width);

  /* The popover is at least as tall as 2 menu items */
  min-height: 6em;
}

6. DOM Interfaces

6.1. The CSSPositionTryRule interface

The CSSPositionTryRule interface represents the @position-try rule:

[Exposed=Window]
interface CSSPositionTryRule : CSSRule {
  readonly attribute CSSOMString name;
  [SameObject, PutForwards=cssText] readonly attribute CSSPositionTryDescriptors style;
};

[Exposed=Window]
interface CSSPositionTryDescriptors : CSSStyleDeclaration {
  attribute CSSOMString margin;
  attribute CSSOMString marginTop;
  attribute CSSOMString marginRight;
  attribute CSSOMString marginBottom;
  attribute CSSOMString marginLeft;
  attribute CSSOMString marginBlock;
  attribute CSSOMString marginBlockStart;
  attribute CSSOMString marginBlockEnd;
  attribute CSSOMString marginInline;
  attribute CSSOMString marginInlineStart;
  attribute CSSOMString marginInlineEnd;
  attribute CSSOMString margin-top;
  attribute CSSOMString margin-right;
  attribute CSSOMString margin-bottom;
  attribute CSSOMString margin-left;
  attribute CSSOMString margin-block;
  attribute CSSOMString margin-block-start;
  attribute CSSOMString margin-block-end;
  attribute CSSOMString margin-inline;
  attribute CSSOMString margin-inline-start;
  attribute CSSOMString margin-inline-end;
  attribute CSSOMString inset;
  attribute CSSOMString insetBlock;
  attribute CSSOMString insetBlockStart;
  attribute CSSOMString insetBlockEnd;
  attribute CSSOMString insetInline;
  attribute CSSOMString insetInlineStart;
  attribute CSSOMString insetInlineEnd;
  attribute CSSOMString top;
  attribute CSSOMString left;
  attribute CSSOMString right;
  attribute CSSOMString bottom;
  attribute CSSOMString inset-block;
  attribute CSSOMString inset-block-start;
  attribute CSSOMString inset-block-end;
  attribute CSSOMString inset-inline;
  attribute CSSOMString inset-inline-start;
  attribute CSSOMString inset-inline-end;
  attribute CSSOMString width;
  attribute CSSOMString minWidth;
  attribute CSSOMString maxWidth;
  attribute CSSOMString height;
  attribute CSSOMString minHeight;
  attribute CSSOMString maxHeight;
  attribute CSSOMString blockSize;
  attribute CSSOMString minBlockSize;
  attribute CSSOMString maxBlockSize;
  attribute CSSOMString inlineSize;
  attribute CSSOMString minInlineSize;
  attribute CSSOMString maxInlineSize;
  attribute CSSOMString min-width;
  attribute CSSOMString max-width;
  attribute CSSOMString min-height;
  attribute CSSOMString max-height;
  attribute CSSOMString block-size;
  attribute CSSOMString min-block-size;
  attribute CSSOMString max-block-size;
  attribute CSSOMString inline-size;
  attribute CSSOMString min-inline-size;
  attribute CSSOMString max-inline-size;
  attribute CSSOMString placeSelf;
  attribute CSSOMString alignSelf;
  attribute CSSOMString justifySelf;
  attribute CSSOMString place-self;
  attribute CSSOMString align-self;
  attribute CSSOMString justify-self;
  attribute CSSOMString positionAnchor;
  attribute CSSOMString position-anchor;
  attribute CSSOMString positionArea;
  attribute CSSOMString position-area;
};

Its name attribute represents the name declared in the rule’s prelude.

Its style attribute represents the properties declared in the rule’s body, in the specified order. On getting, it must return a CSSPositionTryDescriptors object for the @position-try at-rule, with the following properties:

computed flag

Unset

readonly flag

Unset

declarations

The declared descriptors in the rule, in specified order.

parent CSS rule

The context object

owner node

Null

7. Appendix: Style & Layout Interleaving

Style & layout interleaving is a technique where a style update can occur on a subtree during the layout process, resulting in retroactive updates to elements’ computed styles.

This is not the correct spec for this concept, it should probably go in Cascade, but I need a sketch of it to refer to.

Note: Style & layout interleaving is already used with container queries and container query lengths. A length like 10cqw is resolved into a computed length using layout information about the query container’s size, which can thus trigger transitions when the container changes size between layouts.

The accepted @position-try properties are also interleaved when resolving fallback (see position-try).

Obviously this needs way more details filled in, but for now "act like you already do for container queries" suffices. That behavior is also undefined, but at least it’s interoperable (to some extent?).

8. Security Considerations

No Security issues have been raised against this document.

9. Privacy Considerations

No Privacy issues have been raised against this document.

10. Changes

Significant changes since the 26 March 2024 Working Draft:

• Renamed inset-area to position-area

• Added position-anchor: auto to use the implicit anchor element as the default anchor element

• Added top-layer details to the acceptable anchor element definition

• Removed the implicit keyword from the anchor functions (omitting an anchor name does the same thing)

• Removed the "reduce the size of the available space" behavior from anchor-center, now that Align defines how to shift to avoid overflow.

• Added position-visibility, to hide the positioned element in some cases.

• Added a11y guidance (§ 3.7 Accessibility Implications)

• Expanded the set of properties that anchor-size() is used in (to all the relevant accepted @position-try properties).

• Renamed position-try-options to position-try-fallbacks

• Removed the inset-area() function from position-try-fallbacks (you just use a position-area value directly now).

• Defined how to swap due to a try-tactic.

• Expanded the details of how to determine position fallback styles.