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HTML

              
                <div class="controls">
  <fieldset>
    <legend>Position</legend>
    <div>
      <input type="checkbox" class="tl" checked />
      <input type="checkbox" class="tc" />
      <input type="checkbox" class="tr" checked />
    </div>    
    <div>
      <input type="checkbox" class="cl" />
      <input type="checkbox" class="cc" />
      <input type="checkbox" class="cr" />
    </div>    
    <div>
      <input type="checkbox" class="bl" />
      <input type="checkbox" class="bc" />
      <input type="checkbox" class="br" />
    </div>    
  </fieldset>
  <fieldset>
    <legend>inset-inline</legend>
    <div><label>
      <input type="radio" name="inset-inline" class="inset-inline-center" checked />
      auto
    </label></div>
    <div><label>
      <input type="radio" name="inset-inline" class="inset-inline-0" />
      0
    </label></div>
    <div><label>
      <input type="radio" name="inset-inline" class="inset-inline-25" />
      25%
    </label></div>
    <div><label>
      <input type="radio" name="inset-inline" class="inset-inline-50" />
      50%
    </label></div>
  </fieldset>
  <fieldset>
    <legend>inset-block</legend>
    <div><label>
      <input type="radio" name="inset-block" class="inset-block-center" checked />
      auto
    </label></div>
    <div><label>
      <input type="radio" name="inset-block" class="inset-block-0" />
      0
    </label></div>
    <div><label>
      <input type="radio" name="inset-block" class="inset-block-25" />
      25%
    </label></div>
    <div><label>
      <input type="radio" name="inset-block" class="inset-block-50" />
      50%
    </label></div>
  </fieldset>

  <fieldset>
    <legend>margin</legend>
    <div><label>
      <input type="radio" name="margin" class="margin-0" />
      0
    </label></div>
    <div><label>
      <input type="radio" name="margin" class="margin-1" />
      1em
    </label></div>
    <div><label>
      <input type="radio" name="margin" class="margin-2" checked />
      2em
    </label></div>
  </fieldset>
  <fieldset class="code-placeholder">
    <legend>code</legend>
  </fieldset>
  
</div>

<div class="root" style="--is: --root;">
  <div class="resizer" id="resizer"></div>
  <div class="anchor" anchor="resizer" style="--is: --anchor;">
    anchor
  </div>
</div>

<div style="--for: --anchor; --context: --root;">
  <div class="outline outline-top"></div>
  <div class="outline outline-bottom"></div>
  <div class="outline outline-left"></div>
  <div class="outline outline-right"></div>
</div>

<div
  class="popover"
  style="
    --is: --popover;
    --for: --anchor;
    --context: --root;
  "
>
  <div class="popover-content">
    <div contenteditable>this is a popover</div>
  </div>

  <div class="tether tether-center-x"></div>
  <div class="tether tether-center-y"></div>
  <div class="tether tether-corner"><span>tether</span></div>
</div>


<dialog open id="fallback">
  <p>Your browser does not support anchor positioning.</p>
  <p>To play with this demo, open it in Chrome Canary with the “Experimental web platform features” feature flag on.</p>
  <p>Here is a screen recording of this demo:</p>
  <video controls muted playsinline preload="none" poster="https://kizu.dev/videos/inset-area.jpg">
    <source src="https://kizu.dev/videos/inset-area.mp4" type="video/mp4">
    Sorry, your browser doesn’t support embedded videos. You can <a href="https://kizu.dev/videos/inset-area.mp4">download it</a>.
  </video>  
  <p><button type="button" onclick="fallback.close()">Close this dialog</button></p>
</dialog>


              
            
!

CSS

              
                /* The code is a mess! */

.anchor {
  position: absolute;
  top: anchor(bottom);
  left: anchor(right);
  width: 20em;
  height: 15em;
  overflow: hidden;
  resize: both;
  anchor-name: var(--is);
}

/* Move it to the content part for proper tethering */
.popover-content {
  /* Would need to be unique to be able to connect it to both elements */
  anchor-name: var(--is);
  max-width: 100%;
  min-width: 0;
  max-height: 100%;
  min-height: 0;
  pointer-events: auto;
}

.popover {
  position: absolute;
  anchor-default: var(--for);
  transition: all 1s;
  pointer-events: none;
  
  /* Space toggles ftw https://kizu.dev/cyclic-toggles/ */
  --inset-area:
    var(--center-x)
    var(--center-y);
  --left: var(--inset-area,);
  --right: var(--inset-area,);
  --top: var(--inset-area,);
  --bottom: var(--inset-area,);
  --center-x: var(--inset-area,);
  --center-y: var(--inset-area,);
  --all-x: var(--inset-area,);
  --all-y: var(--inset-area,);

  --inset-left: var(--inset-inline, var(--inset, 0));
  --inset-top: var(--inset-block, var(--inset, 0));
  --inset-right: var(--inset-inline, var(--inset, 0));
  --inset-bottom: var(--inset-block, var(--inset, 0));
  

  /* anchor-center */
  --adjust-x: 0px;
  --adjust-y: 0px;
  --anchor-center: min(anchor(auto-same), max(0px, (anchor(auto-same) - anchor(var(--context) auto-same)) - (anchor(var(--context) auto) - anchor(auto))));

  --same-side: anchor(var(--context) auto-same) +
    (anchor(auto-same) - anchor(var(--context) auto-same));
  --start-center-side: anchor(auto-same) + 
    (anchor(auto) - anchor(auto-same));
  --start-full-side: anchor(auto) +
    (anchor(var(--context) auto) - anchor(auto));
  --end-center-side: anchor(auto-same) - 
    (anchor(auto-same) - anchor(auto));
  --end-full-side: anchor(auto) -
    (anchor(auto) - anchor(var(--context) auto));

  --top-center-no-top:
    var(--top)
    var(--center-y, var(--start-center-side));
  --top-bottom-no-center:
    var(--center-y)
    var(--bottom, var(--start-full-side));
  top: calc(
    var(--adjust-y) + 
    var(--top, var(--same-side))
    var(--top-bottom-no-center,)
    var(--top-center-no-top,)
    var(--all-y, var(--same-side))
    * var(--inset-top)
  );
  --bottom-top-no-center:
    var(--center-y)
    var(--top, var(--end-full-side));
  --bottom-center-no-bottom:
    var(--bottom)
    var(--center-y, var(--end-center-side));
  bottom: calc(
    var(--adjust-y) + 
    var(--bottom-top-no-center,)
    var(--bottom, var(--same-side))
    var(--bottom-center-no-bottom,)
    var(--all-y, var(--same-side))
    * var(--inset-bottom)
  );
  --left-center-no-left:
    var(--left)
    var(--center-x, var(--start-center-side));
  --left-right-no-center:
    var(--center-x)
    var(--right, var(--start-full-side));
  left: calc(
    var(--adjust-x) +
    var(--left, var(--same-side))
    var(--left-right-no-center,)
    var(--left-center-no-left,)
    var(--all-x, var(--same-side))
    * var(--inset-left)
  );
  --right-left-no-center:
    var(--center-x)
    var(--left, var(--end-full-side));
  --right-center-no-right:
    var(--right)
    var(--center-x, var(--end-center-side));
  right: calc(
    var(--adjust-x) + 
    var(--right-left-no-center,)
    var(--right, var(--same-side))
    var(--right-center-no-right,)
    var(--all-x, var(--same-side))
    * var(--inset-right)
  );
  
}

.tether {
  pointer-events: none;
  position: fixed;
  anchor-default: var(--for);
}

.tether-center-x {
  anchor-name: --center-x;
  inset-block: min(anchor(auto), anchor(var(--is) auto));
  inset-inline: max(anchor(auto-same), anchor(var(--is) auto-same));
}
.tether-center-y {
  anchor-name: --center-y;
  inset-block: max(anchor(auto-same), anchor(var(--is) auto-same));
  inset-inline: min(anchor(auto), anchor(var(--is) auto));
}

.tether-corner {
  border: 4px dotted;
  anchor-default: var(--for);
  inset: min(anchor(--center-x auto-same), anchor(--center-y auto-same));
}

/* Configurable */

body:has(.inset-inline-0:checked) .popover {
  --inset-inline: 0;
  --adjust-x: 0px !important;
}

body:has(.inset-inline-25:checked) .popover {
  --inset-inline: 0.25;
  --adjust-x: 0px !important;
}

body:has(.inset-inline-50:checked) .popover {
  --inset-inline: 0.5;
  --adjust-x: 0px !important;
}

body:has(.inset-block-0:checked) .popover {
  --inset-block: 0;
  --adjust-y: 0px !important;
}

body:has(.inset-block-25:checked) .popover {
  --inset-block: 0.25;
  --adjust-y: 0px !important;
}

body:has(.inset-block-50:checked) .popover {
  --inset-block: 0.5;
  --adjust-y: 0px !important;
}

body:has(.margin-0:checked) .popover {
  margin: 0;
}

body:has(.margin-1:checked) .popover {
  margin: 1em
}

body:has(.margin-2:checked) .popover {
  margin: 2em;
}


/* 1x1 */
body:has(.tl:checked) .popover {
  --code: 'top / left';
  --inset-area: var(--top) var(--left);
  --align: var(--align-bottom) var(--align-right);
}

body:has(.tc:checked) .popover {
  --code: 'top / center';
  --inset-area: var(--top) var(--center-x);
  --align: var(--align-bottom) var(--align-center-x);
}

body:has(.tr:checked) .popover {
  --code: 'top / right';
  --inset-area: var(--top) var(--right);
  --align: var(--align-bottom) var(--align-left);
}

body:has(.cl:checked) .popover {
  --code: 'center / left';
  --inset-area: var(--center-y) var(--left);
  --align: var(--align-center-y) var(--align-right);
}

body:has(.cr:checked) .popover {
  --code: 'center / right';
  --inset-area: var(--center-y) var(--right);
  --align: var(--align-center-y) var(--align-left);
}

body:has(.bl:checked) .popover {
  --code: 'bottom / left';
  --inset-area: var(--bottom) var(--left);
  --align: var(--align-top) var(--align-right);
}

body:has(.bc:checked) .popover {
  --code: 'bottom / center';
  --inset-area: var(--bottom) var(--center-x);
  --align: var(--align-top) var(--align-center-x);
}

body:has(.br:checked) .popover {
  --code: 'bottom / right';
  --inset-area: var(--bottom) var(--right);
  --align: var(--align-top) var(--align-left);
}


/* 1x2 */
body:has(.tl:checked):has(.tc:checked) .popover {
  --code: 'top / left center';
  --inset-area: var(--top) var(--left) var(--center-x);
  --align: var(--align-bottom) var(--align-center-x);
}

body:has(.tr:checked):has(.tc:checked) .popover {
  --code: 'top / right center';
  --inset-area: var(--top) var(--right) var(--center-x);
  --align: var(--align-bottom) var(--align-center-x);
}

body:has(.bl:checked):has(.bc:checked) .popover {
  --code: 'bottom / left center';
  --inset-area: var(--bottom) var(--left) var(--center-x);
  --align: var(--align-top) var(--align-center-x);
}

body:has(.br:checked):has(.bc:checked) .popover {
  --code: 'bottom / right center';
  --inset-area: var(--bottom) var(--right) var(--center-x);
  --align: var(--align-top) var(--align-center-x);
}

body:has(.tl:checked):has(.cl:checked) .popover {
  --inset-area: var(--top) var(--center-y) var(--left);
  --code: 'top center / left';
  --align: var(--align-center-y) var(--align-right);
}

body:has(.tr:checked):has(.cr:checked) .popover {
  --inset-area: var(--top) var(--center-y) var(--right);
  --code: 'top center / right';
  --align: var(--align-center-y) var(--align-left);
}

body:has(.bl:checked):has(.cl:checked) .popover {
  --inset-area: var(--bottom) var(--center-y) var(--left);
  --code: 'bottom center / left';
  --align: var(--align-center-y) var(--align-right);
}

body:has(.br:checked):has(.cr:checked) .popover {
  --inset-area: var(--bottom) var(--center-y) var(--right);
  --code: 'bottom center / right';
  --align: var(--align-center-y) var(--align-left);
}

/* cc */
body:has(.tc:checked):has(.cc:checked) .popover {
  --inset-area: var(--top) var(--center-y) var(--center-x);
  --code: 'top center / center';
  --align: var(--align-center-y) var(--align-center-x);
}
body:has(.bc:checked):has(.cc:checked) .popover {
  --inset-area: var(--bottom) var(--center-y) var(--center-x);
  --code: 'bottom center / center';
  --align: var(--align-center-y) var(--align-center-x);
}
body:has(.cl:checked):has(.cc:checked) .popover {
  --inset-area: var(--center-y) var(--left) var(--center-x);
  --code: 'center / left center';
  --align: var(--align-center-y) var(--align-center-x);
}
body:has(.cr:checked):has(.cc:checked) .popover {
  --inset-area: var(--center-y) var(--right) var(--center-x);
  --code: 'center / right center';
  --align: var(--align-center-y) var(--align-center-x);
}


/* 2x2 */
body:has(.tc:checked):has(.cl:checked) .popover,
body:has(.tl:checked):has(.cc:checked) .popover {
  --inset-area: var(--top) var(--center-y) var(--left) var(--center-x);
  --code: 'top center / left center';
  --align: var(--align-center-y) var(--align-center-x);
}

body:has(.tc:checked):has(.cr:checked) .popover,
body:has(.tr:checked):has(.cc:checked) .popover {
  --inset-area: var(--top) var(--center-y) var(--right) var(--center-x);
  --code: 'top center / right center';
  --align: var(--align-center-y) var(--align-center-x);
}

body:has(.bc:checked):has(.cl:checked) .popover,
body:has(.bl:checked):has(.cc:checked) .popover {
  --inset-area: var(--bottom) var(--center-y) var(--left) var(--center-x);
  --code: 'bottom center / left center';
  --align: var(--align-center-y) var(--align-center-x);
}

body:has(.bc:checked):has(.cr:checked) .popover,
body:has(.br:checked):has(.cc:checked) .popover {
  --inset-area: var(--bottom) var(--center-y) var(--right) var(--center-x);
  --code: 'bottom center / right center';
  --align: var(--align-center-y) var(--align-center-x);
}


/* 1x3 */
body:has(.tl:checked):has(.tr:checked) .popover {
  --inset-area: var(--top) var(--all-x);
  --code: 'top / all';
  --align: var(--align-bottom) var(--align-center-x);
  --adjust-x: var(--anchor-center);
}
body:has(.bl:checked):has(.br:checked) .popover {
  --inset-area: var(--bottom) var(--all-x);
  --code: 'bottom / all';
  --align: var(--align-top) var(--align-center-x);
  --adjust-x: var(--anchor-center);
}
body:has(.tl:checked):has(.bl:checked) .popover {
  --inset-area: var(--all-y) var(--left);
  --code: 'all / left';
  --align: var(--align-center-y) var(--align-right);
  --adjust-y: var(--anchor-center);
}
body:has(.tr:checked):has(.br:checked) .popover {
  --inset-area: var(--all-y) var(--right);
  --code: 'all / right';
  --align: var(--align-center-y) var(--align-left);
  --adjust-y: var(--anchor-center);
}

/* center 1x3 */
body:has(.tc:checked):has(.bc:checked) .popover {
  --inset-area: var(--all-y) var(--center-x);
  --code: 'all / center';
  --align: var(--align-center-y) var(--align-center-x);
  --adjust-y: var(--anchor-center);
}

body:has(.cl:checked):has(.cr:checked) .popover {
  --inset-area: var(--center-y) var(--all-x);
  --code: 'center / all';
  --align: var(--align-center-y) var(--align-center-x);
  --adjust-x: var(--anchor-center);
}


/* 2/3 */
body:has(.cl:checked):has(.tr:checked) .popover,
body:has(.cr:checked):has(.tl:checked) .popover,
body:has(.cl:checked):has(.cr:checked):has(.tc:checked) .popover,
body:has(.tl:checked):has(.tr:checked):has(.cc:checked) .popover {
  --inset-area: var(--top) var(--center-y) var(--all-x);
  --code: 'top center / all';
  --align: var(--align-center-y) var(--align-center-x);
  --adjust-x: var(--anchor-center);
}

body:has(.cl:checked):has(.br:checked) .popover,
body:has(.cr:checked):has(.bl:checked) .popover,
body:has(.cl:checked):has(.cr:checked):has(.bc:checked) .popover,
body:has(.bl:checked):has(.br:checked):has(.cc:checked) .popover {
  --inset-area: var(--bottom) var(--center-y) var(--all-x);
  --code: 'bottom center / all';
  --align: var(--align-center-y) var(--align-center-x);
  --adjust-x: var(--anchor-center);
}

body:has(.tc:checked):has(.bl:checked) .popover,
body:has(.bc:checked):has(.tl:checked) .popover,
body:has(.tc:checked):has(.bc:checked):has(.cl:checked) .popover,
body:has(.tl:checked):has(.bl:checked):has(.cc:checked) .popover {
  --inset-area: var(--all-y) var(--left) var(--center-x);
  --code: 'all / left center';
  --align: var(--align-center-y) var(--align-center-x);
  --adjust-y: var(--anchor-center);
}

body:has(.tc:checked):has(.br:checked) .popover,
body:has(.bc:checked):has(.tr:checked) .popover,
body:has(.tc:checked):has(.bc:checked):has(.cr:checked) .popover,
body:has(.tr:checked):has(.br:checked):has(.cc:checked) .popover {
  --inset-area: var(--all-y) var(--right) var(--center-x);
  --code: 'all / right center';
  --align: var(--align-center-y) var(--align-center-x);
  --adjust-y: var(--anchor-center);
}


/* 3x3 */
body:has(.tl:checked):has(.br:checked) .popover,
body:has(.tr:checked):has(.bl:checked) .popover,
body:has(:is(.tl, .tc, .tr):checked):has(:is(.cl, .cc, .cr):checked):has(:is(.bl, .bc, .br):checked):has(:is(.tl, .cl, .bl):checked):has(:is(.tr, .cr, .br):checked) .popover,
body:has(:is(.tl, .cl, .bl):checked):has(:is(.tc, .cc, .bc):checked):has(:is(.tr, .cr, .br):checked):has(:is(.tl, .tc, .tr):checked):has(:is(.bl, .bc, .br):checked) .popover{
  --inset-area: var(--all-y) var(--all-x);
  --code: 'all / all';
  --align: var(--align-center-y) var(--align-center-x);
  --adjust-x: var(--anchor-center);
  --adjust-y: var(--anchor-center);
}




/* Presentational Styles */

body {
  margin: 0;
  font-family: sans-serif;
}

.anchor,
.popover-content {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  color: #FFF;
  border-radius: 1em;
}

.anchor {
  background: #f7b722;
  
}
.popover {
  display: grid;
  --align: var(--align-center-y) var(--align-center-x);
  --align-center-y: var(--align,);
  --align-center-x: var(--align,);
  --align-left: var(--align,);
  --align-right: var(--align,);
  --align-top: var(--align,);
  --align-bottom: var(--align,);
  --left-align:
    var(--align-left, 0fr)
    var(--align-center-x, 1fr)
    var(--align-right, 1fr);
  --right-align:
    var(--align-left, 1fr)
    var(--align-center-x, 1fr)
    var(--align-right, 0fr);
  --top-align:
    var(--align-top, 0fr)
    var(--align-center-y, 1fr)
    var(--align-bottom, 1fr);
  --bottom-align:
    var(--align-top, 1fr)
    var(--align-center-y, 1fr)
    var(--align-bottom, 0fr);

  grid-template-columns: var(--left-align) auto var(--right-align);
  grid-template-rows: var(--top-align) auto var(--bottom-align);
  background: #ffc0cba3;
  transition: all 1s;
}

.popover-content {
  padding: 0.5em;
  transition: all 1s;
  grid-area: 2 / 2;
  place-self: var(--align, center center);
  background: #ff40ff;
  min-height: auto;
  min-width: auto;
}

fieldset {
  padding: 0.5em;
}

.code-placeholder {
  flex-grow: 1;
  anchor-name: --code-placeholder;
}

.popover::after {
  position: fixed;
  inset: calc(anchor(auto-same) + 0.5em);
  top: 1.5em;
  color: #000;
  content: 'inset-area: ' var(--code, 'center / center') ';';
  font-family: monospace;
  anchor-default: --code-placeholder;
}

.tether {
  display: flex;
  align-items: center;
  justify-content: center;
  color: #000;
  z-index: -1; /* Not working well when intersecting */
  container-type: size;
  font-size: 0.75em;
}

@container (aspect-ratio < 1) {
  .tether > span {
    writing-mode: tb;
  }
}

body {
  display: grid;
  min-height: 100vh;
  grid-template-rows: max-content 1fr;
}

.controls {
  display: flex;
  gap: 0.5em;
  white-space: nowrap;
}

.root {
  anchor-name: var(--is);
  box-shadow: 0 -2px 0 #0003;
  overflow: hidden;
  resize: both;
}

.resizer {
  width: 15em;
  height: 10em;
  overflow: hidden;
  resize: both;
}

.outline {
  z-index: -2;
  position: fixed;
  outline: 1px solid #0004;
  pointer-events: none;
  anchor-default: var(--for);
}

.outline-top {
  top: anchor(top);
  inset-inline: anchor(var(--context) auto-same);
}
.outline-bottom {
  bottom: anchor(bottom);
  inset-inline: anchor(var(--context) auto-same);
}
.outline-left {
  left: anchor(left);
  inset-block: anchor(var(--context) auto-same);
}
.outline-right {
  right: anchor(right);
  inset-block: anchor(var(--context) auto-same);
}

dialog {
  z-index: 9;
  outline: 200vmax solid #1238;
  overflow: auto;
  max-height: 100vh;
  box-sizing: border-box;  
}

video {
  max-width: 100%;
  max-height: 100vh;
  width: 861px;
  height: 540px;
}

@supports (anchor-name: --foo) {
  dialog {
    display: none;
  }
}

              
            
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JS

              
                
              
            
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999px

Console