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CSS

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <div class="resizer"></div>

<button id="button" type="button" popovertarget="popover">
  Discover Winter Wonders
  <sup>❄︎</sup>
</button>

<div anchor="button" id="popover" popover>❄︎ The presence of snow does not mean climate change is not happening.</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/anchor-positioning-button-dark.jpg">
    <source src="https://kizu.dev/videos/anchor-positioning-button-dark.mp4" type="video/mp4" media="(prefers-color-scheme: dark)">
    <source src="https://kizu.dev/videos/anchor-positioning-button-light.mp4" type="video/mp4">
    Sorry, your browser doesn’t support embedded videos. You can <a href="https://kizu.dev/videos/anchor-positioning-button-dark.mp4">download it</a>.
  </video>
  <p><button type="button" onclick="fallback.close()">Close this dialog</button></p>
</dialog>
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CSS 

              
                @import url('https://fonts.googleapis.com/css2?family=Playpen+Sans:wght@100..800&display=swap');


button {
  position: relative;
  cursor: pointer;
  grid-area: --button;
  font: inherit;
  border: 1px solid transparent;
  box-shadow: 1px 2px 2px 1px forestgreen;
  color: #000;
  background: lightgreen;
  border-radius: 9em;
}

sup {
  position: absolute;
  inset-block-start: -0.25ex;
  inset-inline-end: -0.75ex;
  line-height: 1ex;
  font-size: 1.5em;
  anchor-name: --sup;

  @media (prefers-color-scheme: dark) {
    color: #FFF;
    font-weight: bold;
  }
}

[popover] {
  padding: 0.5em;
  border-radius: 0.25em;
  text-align: center;

  min-width: anchor-size(inline);
  max-width: 20em;
  anchor-name: --popover;
  grid-area: --fallback;
    
  &::before,
  &::after {
    content: "";
    position: fixed;
    width: anchor-size(inline);
    position-anchor: --sup;
    background: inherit;
    inset-inline: anchor(start) anchor(end);
    max-height: anchor-size(inline);
    margin: auto;
    inset-block-start: calc(anchor(end) + 0.5em);
    inset-block-end: calc(anchor(--popover start) - 0.5em);
    transform: translateY(16.666%) rotate(45deg);
    box-shadow: inset 1.5px 1.5px;
  }
  &::after {
    inset-block-start: calc(anchor(--popover end) - 0.5em);
    inset-block-end: calc(anchor(start));
    transform: translateY(calc(-16.666%)) rotate(45deg);
    box-shadow: inset -1.5px -1.5px;
  }
  
  &::backdrop {
    position: absolute;
    inset: anchor(start) anchor(end) anchor(end) anchor(start);
    border-radius: 9em;
    outline: 100vmax solid #0074
  }
}

@supports (anchor-name: --foo) {
  [popover] {
    /* Old anchor-center algorithm */
    inset-inline: calc(
      anchor(50%)
      -
      min(
        abs(0% - anchor(50%)),
        abs(100% - anchor(50%))
      )
    );
    margin-inline: auto;
    position-try: flip-block;
    inset-block-start: auto;
    inset-block-end: max(anchor(start), anchor(--sup start) + 0.5em);
    margin-block: 0.25em;
  }
}

/* Page and resizer setup */

html {
  color-scheme: dark light;
}

body {
  margin: 0;
  min-height: 100vh;
  display: grid;
  --resizer-area: 1fr;
  grid-template:
    '--resizer . .' var(--resizer-area)
    '. --button .' max-content
    '. --fallback .' 1fr / var(--resizer-area) max-content 1fr;
  place-items: center;
  font-family: Playpen Sans, sans-serif;
}

body:has(.resizer[style]) {
  --resizer-area: max-content;
}

.resizer {
  overflow: hidden;
  resize: both;
  width: 100%;
  height: 100%;
  grid-area: --resizer;
}

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: 722px;
  height: 414px;
}

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

JS
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999px

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