inset-area

The property value is two <inset-area> keyterms, or the keyword none. If only one <inset-area> keyterm is provided, the second keyterm is implied.

<inset-area>

Specifies the area of the inset area grid on which to place selected positioned elements.

none

No inset area is set.

The inset-area property provides an alternative to the anchor() function for positioning elements relative to anchors. inset-area works on the concept of a 3x3 grid of tiles, called the inset-area grid, with the anchor element being the center tile:

The grid tiles are broken up into rows and columns:

• The three rows are represented by the physical values top, center, and bottom. They also have logical equivalents such as block-start, center, and block-end, and coordinate equivalents — y-start, center, and y-end.
• The three columns are represented by the physical values left, center, and right. They also have logical equivalents such as inline-start, center, and inline-end, and coordinate equivalents — x-start, center, and x-end.

The dimensions of the center tile are defined by the containing block of the anchor element, while the dimensions of the grid's outer edge are defined by the positioned element's containing block.

The <inset-area> value is composed of one or two keywords, which define the region of the grid the positioned element should be placed inside. To be exact, the containing block of the positioned element is set to the grid area.

For example:

• You can specify a row value and a column value to place the positioned element in a single, specific grid square — for example, top left (logical equivalent start start) or bottom center (logical equivalent end center) will place the positioned element in the top-right or bottom-center square.
• You can specify a row or column value plus a span-* value to span two or three cells. The first value specifies the row or column to place the positioned element in, placing it initially in the center, and the other one specifies the other tiles of that row or column to span. For example:
  • top span-left causes the positioned element to be placed in the center of the top row, and span across the center and left tiles of that row.
  • block-end span-inline-end causes the positioned element to be placed in the center of the block end row, and span across the center and inline end tiles of that row.
  • bottom span-all and y-end span-all cause the positioned element to be placed in the center of the bottom row, and span across three cells, in this case the left, center, and right tiles of the bottom row.

For detailed information on anchor features, usage, and the inset-area property, see the CSS anchor positioning module landing page and the Using CSS anchor positioning guide, specifically the section on setting an inset-area.

When an <inset-area> value is set on a positioned element, some of its properties will have their default behavior adjusted to provide a good default alignment.

Self-alignment property normal value

The normal value of the self-alignment properties, including align-items, align-self, justify-items, and justify-self, behaves as either start, end, or anchor-center. Which value the self-alignment property defaults to depends on the positioning of the element:

• If the inset-area value specifies the center region in an axis, the default alignment in that axis is anchor-center.
• Otherwise, the behavior is the opposite of the region specified by the inset-area property. For example, if the inset-area value specifies the start region of its axis, the default alignment in that axis is end.

For example, if the writing-mode is set to horizontal-tb, inset-area: top span-x-start causes the positioned element to be placed in the center of the top row, and span across the center and start tiles of that row. In this case, the self-alignment properties will default to align-self: end and justify-self: anchor-center.

inset properties and values

When an anchor-positioned element is positioned using the inset-area property, any inset properties set, such as top or inset-inline-end, specify offsets from the inset-area. Some other property values, like max-block-size: 100%, will also be relative to the inset-area. Any inset properties set or defaulting to auto will behave as if their value was set to 0.

If the positioned element does not have a specific size set on it, its size will default to its intrinsic size, but it will also be affected by the size of the inset-area grid.

If the positioned element is placed in a single top-center, bottom-center, or center-center cell, its block size will be the same as the anchor's containing block size, growing up, down, or in both directions, respectively, as needed. The positioned element will align with the specified grid square but adopt the same width as the anchor element. However, it won't allow its content to overflow — its minimum width will be its min-content (as defined by the width of its longest word).

If the positioned element is placed in any other single grid square (say with inset-area: top left) or is set to span two or more grid squares (for example using inset-area: bottom span-all), it will align with the specified grid area but behave as if it has a width of max-content set on it. It is being sized according to its containing block size, which is the size imposed on it when it was set to position: fixed. It will stretch as wide as the text content, although it may also be constrained by the edge of the <body>.

inset-area = 
  none          |
  <inset-area>  

<inset-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}  

In this example, a positioned element is tethered and positioned relative to its associated anchor using the inset-area property.

HTML

The HTML includes a <div> and a <p>. The <p> will be positioned relative to the <div> with CSS. We also include a style block that will be made visible. All elements are set to be directly editable via the contenteditable attribute.

<div class="anchor" contenteditable="true">⚓︎</div>

<p class="positionedElement" contenteditable="true">This can be edited.</p>

<style contenteditable="true">.positionedElement {
    inset-area: CHANGEME;
  }
</style>

CSS

We convert the <div> to an anchor element with the anchor-name property. We then associate the absolutely-positioned <p> with it by setting its position-anchor value to the same anchor name.

We set the initial inset-area value to top center. This value is set on a p selector, so the value has less specificity than any value added to the <style> block's .positionedElement class selector. As a result, you can override the initial inset-area value by setting an inset-area value inside the style block.

.anchor {
  anchor-name: --infobox;
  background: palegoldenrod;
  font-size: 3em;
  width: fit-content;
  border: 1px solid goldenrod;
  margin: 100px auto;
}

p {
  position: absolute;
  position-anchor: --infobox;
  inset-area: top center;
  margin: 0;
  background-color: darkkhaki;
  border: 1px solid darkolivegreen;
}

style {
  display: block;
  white-space: pre;
  font-family: monospace;
  background-color: #ededed;
  -webkit-user-modify: read-write-plaintext-only;
  line-height: 1.5;
  padding: 10px;
}

Results

Try changing the amount of text in the anchor-positioned element to see how it grows. Also, try changing the invalid "CHANGEME" value of the inset-area property to a valid value.

Let's show some inset-area values in action. This demo creates an anchor and tethers a positioned element to it. It also provides a drop-down menu allowing you to choose various inset-area values to apply to the positioned element, to see their effect. One of the options causes a text field to appear that enables you to enter a custom value. Finally, a checkbox is provided to turn writing-mode: vertical-lr on and off, allowing you to observe how inset-area value effects differ across different writing modes.

HTML

In the HTML, we specify two <div> elements, one with a class of anchor and one with a class of infobox. These are intended to be the anchor element and the positioned element we will associate with it, respectively. We've included the contenteditable attribute on both, making them directly editable.

We've also included two forms that contain the <select> and <input type="text"> elements for setting different inset-area values, and the <input type="checkbox"> element for toggling the vertical writing-mode on and off. The code for these, along with the JavaScript, has been hidden for the sake of brevity.

<div class="anchor" contenteditable>⚓︎</div>

<div class="infobox">
  <p contenteditable>You can edit this text.</p>
</div>

<form id="inset-area-form">
  <div>
    <label for="inset-area-select">Choose an inset-area:</label>
    <select id="inset-area-select" name="inset-area-select">
      <optgroup label="Enter custom value">
        <option>Custom</option>
      </optgroup>
      <optgroup label="Physical, single tile">
        <option selected>top left</option>
        <option>bottom right</option>
      </optgroup>
      <optgroup label="Physical, span two">
        <option>bottom span-right</option>
        <option>left span-top</option>
      </optgroup>
      <optgroup label="Physical, span all">
        <option>top</option>
        <option>left</option>
      </optgroup>
      <optgroup label="Logical, single tile">
        <option>start center</option>
        <option>inline-start block-end</option>
      </optgroup>
      <optgroup label="Logical, span two">
        <option>start span-end</option>
        <option>center span-start</option>
        <option>inline-start span-block-end</option>
        <option>span-block-start center</option>
      </optgroup>
      <optgroup label="Logical, span all">
        <option>start span-all</option>
        <option>block-end</option>
      </optgroup>
      <optgroup label="Coordinates, single tile">
        <option>x-start center</option>
        <option>x-end y-end</option>
      </optgroup>
      <optgroup label="Coordinates, span two">
        <option>center span-y-start</option>
        <option>y-start span-x-end</option>
      </optgroup>
      <optgroup label="Coordinates, span all">
        <option>x-start span-all</option>
        <option>y-end</option>
      </optgroup>
    </select>
  </div>
  <div id="inset-area-custom-container">
    <label for="inset-area-custom">Enter a custom inset-area:</label><br />
    <input type="text" id="inset-area-custom" name="inset-area-custom" />
  </div>
</form>

<form id="writing-mode-form">
  <label for="writing-mode-checkbox">writing-mode: vertical-lr</label><br />
  <input
    type="checkbox"
    id="writing-mode-checkbox"
    name="writing-mode-checkbox" />
</form>

CSS

In the CSS, we first declare the anchor <div> as an anchor element by setting an anchor name on it via the anchor-name property.

The positioned element is associated with the anchor element by setting its anchor name as the value of the positioned element's position-anchor property. We also give it an initial position with inset-area: top left; this will be overridden when new values are selected from the <select> menu. Finally, we set its opacity to 0.8, so that when the positioned element is given an inset-area value that places it over the top of the anchor, you can still see the elements' position relative to one another.

.anchor {
  font-size: 1.8rem;
  text-align: center;
  color: white;
  text-shadow: 1px 1px 1px black;
  background-color: hsl(240 100% 75%);
  min-width: 50px;
  min-height: 50px;
  line-height: 50px;
  border-radius: 10px;
  border: 1px solid black;
  padding: 3px;
}

html {
  height: 100%;
}

body {
  margin: 0;
  width: 100vw;
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
}

form {
  background: white;
  border: 1px solid black;
  padding: 5px;
  position: fixed;
}

#inset-area-form {
  top: 0;
  right: 2px;
}

#inset-area-form div:last-child {
  margin-top: 10px;
}

#inset-area-form div :last-child {
  margin-top: 5px;
}

#writing-mode-form {
  bottom: 0;
  left: 2px;
  writing-mode: horizontal-tb;
}

#inset-area-custom-container {
  display: none;
}

.infobox {
  color: darkblue;
  background-color: azure;
  border: 1px solid #ddd;
  padding: 5px 2px;
  border-radius: 5px;
  font-size: 1rem;
  box-sizing: border-box;
}

.anchor {
  anchor-name: --myAnchor;
}

.infobox {
  position-anchor: --myAnchor;
  position: fixed;
  opacity: 0.8;
  inset-area: top left;
}

const anchorContainer = document.querySelector("body");
const infobox = document.querySelector(".infobox");

const insetAreaForm = document.querySelector("#inset-area-form");
const selectElem = document.querySelector("select");
const inputElemContainer = document.querySelector(
  "#inset-area-custom-container",
);
const inputElem = document.querySelector("#inset-area-custom");
const checkboxElem = document.querySelector("#writing-mode-checkbox");

// Stop form from submitting when enter is pressed on it
insetAreaForm.addEventListener("submit", (e) => {
  e.preventDefault();
});

// Update positioned element inset-area to value selected in drop-down
selectElem.addEventListener("change", () => {
  const area = selectElem.value;

  if (area === "Custom") {
    // Show the custom value input field
    inputElemContainer.style.display = "block";
  } else {
    // Hide the custom value input field
    inputElemContainer.style.display = "none";
    // Set the inset-area to the value chosen in the select box
    infobox.style.insetArea = area;
  }
});

// Update positioned element inset-area to entered custom value
inputElem.addEventListener("change", () => {
  const customArea = inputElem.value;

  // Set the inset-area to the value in the input box
  infobox.style.insetArea = customArea;
});

// Change writing-mode to match checkbox setting
checkboxElem.addEventListener("change", () => {
  if (checkboxElem.checked) {
    anchorContainer.style.writingMode = "vertical-lr";
  } else {
    anchorContainer.style.writingMode = "horizontal-tb";
  }
});

Result

The result is as follows:

Try selecting new inset-area values from the <select> menu to see the effect they have on the position of the infobox. Select the "Custom" value and try entering some custom inset-area values into the text input to see their effect. Add text to the anchor and the anchor positioned elements to see how the anchor positioned element grows based on the inset-area value. Finally, check the checkbox and then experiment with different inset-area values to see which ones give the same result across different writing modes, and which ones give different results.

• anchor-name
• position-anchor
• position-try-fallbacks
• The anchor() function
• The <inset-area> value
• Using CSS anchor positioning guide
• Handling overflow: try fallbacks and conditional hiding guide
• CSS anchor positioning module