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CSS

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HTML 

              
                 <!-- <svg style="display: none">
  <symbol id="checkmark" viewBox="0 0 24 24">
    <path stroke-linecap="round" stroke-linejoin="round" d="M9 12.75L11.25 15 15 9.75M21 12c0 1.268-.63 2.39-1.593 3.068a3.745 3.745 0 01-1.043 3.296 3.745 3.745 0 01-3.296 1.043A3.745 3.745 0 0112 21c-1.268 0-2.39-.63-3.068-1.593a3.746 3.746 0 01-3.296-1.043 3.745 3.745 0 01-1.043-3.296A3.745 3.745 0 013 12c0-1.268.63-2.39 1.593-3.068a3.745 3.745 0 011.043-3.296 3.746 3.746 0 013.296-1.043A3.746 3.746 0 0112 3c1.268 0 2.39.63 3.068 1.593a3.746 3.746 0 013.296 1.043 3.746 3.746 0 011.043 3.296A3.745 3.745 0 0121 12z" />
  </symbol>
</svg>  -->

<!-- <select>
  <option value="">foo</option>
  <option value="">bar</option>
  <option value="">baz</option>
</select>

<select multiple style="inline-size: 10ch;">
  <option value="">foo</option>
  <option value="">bar</option>
  <option value="">baz</option>
</select>

<select multiple style="inline-size: 10ch;">
  <optgroup label="Group 1">
    <option value="">foo</option>
  </optgroup>
  <optgroup label="Group 2">
    <option value="">bar</option>
    <option value="">baz</option>
  </optgroup>
</select>

<selectlist>
  <option value="">foo</option>
  <option value="">bar</option>
  <option value="">baz</option>
</selectlist>

<selectlist multiple>
  <option value="">foo</option>
  <option value="">bar</option>
  <option value="">baz</option>
</selectlist> --> 

<selectlist>
  <button type=selectlist>
    <div>
      <selectedoption></selectedoption>
      <svg width="24" height="24" viewBox="0 0 24 24">
        <path fill="currentColor" d="m7 10l5 5l5-5z"/>
      </svg>
    </div>
  </button>
  <listbox popover=auto>
    <option>
      <!-- custom so it's fully consumed for selectedoption -->
      <div class="custom-option">
        <span class="indicator success"></span>
        <span class="option-text">On</span>
      </div>
    </option>
    <option>
      <div class="custom-option">
        <span class="indicator danger"></span>
        <span class="option-text">Off</span>
      </div>
    </option>
    <option>
      <div class="custom-option">
        <span class="indicator disable"></span>
        <span class="option-text">Disable</span>
      </div>
    </option>
  </listbox>
</selectlist>

<selectlist>
  <button type=selectlist>
    <div>
      <selectedoption></selectedoption>
      <svg width="24" height="24" viewBox="0 0 24 24">
        <path fill="currentColor" d="m7 10l5 5l5-5z"/>
      </svg>
    </div>
  </button>
  <listbox popover=auto>
    <option>
      <div class="custom-option">
        <span class="avatar">
          <img width="20" height="20" src="https://pbs.twimg.com/profile_images/1587634978308997121/u7009cGe_400x400.jpg" alt="">
        </span>
        <span class="option-text">Una Kravets</span>
      </div>
    </option>
    <option>
      <div class="custom-option">
        <span class="avatar">
          <img width="20" height="20" src="https://pbs.twimg.com/profile_images/1276240813333401600/brd0hSfW_400x400.jpg" alt="">
        </span>
        <span class="option-text">Bramus Van Damme</span>
      </div> 
    </option>
    <option>
      <div class="custom-option">
        <span class="avatar">
          <img width="20" height="20" src="https://pbs.twimg.com/profile_images/1720589781476982784/P9Ld4vC5_400x400.jpg" alt="">
        </span>
        <span class="option-text">Adam Argyle</span>
      </div>
    </option>
  </listbox>
</selectlist>

<selectlist id="colorspace">
  <button type=selectlist>
    <small>Color Space</small>
    <div>
      <selectedoption style="view-transition-name: foo"></selectedoption>
      <svg width="24" height="24" viewBox="0 0 24 24">
        <path fill="currentColor" d="m7 10l5 5l5-5z"/>
      </svg>
    </div>
  </button> 
  <listbox popover=auto>
    <div role="group">
      <label>Standard</label>
      <option value="srgb">rgb</option>
      <option>srgb-linear</option>
      <option>hsl</option>
      <option>hwb</option>
    </div>
    <div role="group">
      <label>HD</label>
      <option>display-p3</option>
      <option>a98-rgb</option>
    </div>
    <div role="group">
      <label>Ultra HD</label>
      <option>lab</option>
      <option>lch</option> 
      <option selected>oklch</option>
      <option>oklab</option>
      <option>rec2020</option>
      <option>prophoto</option>
      <option>xyz</option>
      <option>xyz-d50</option>
      <option>xyz-d65</option>
    </div>
  </listbox>
</selectlist>

<selectlist class="primary">
  <button type=selectlist>
    <div>
      <selectedoption></selectedoption>
      <svg width="24" height="24" viewBox="0 0 24 24">
        <path fill="currentColor" d="m7 10l5 5l5-5z"/>
      </svg>
    </div>
  </button>
  <listbox popover=auto>
    <option>
      <div class="custom-option">
        <div>Published</div>
        <div class="description">Live on the web, available to anyone with the link.</div>
      </div>
    </option>
    <option>
      <div class="custom-option">
        <div>Draft</div>
        <div class="description">Not available on the web, no public access.</div>
      </div> 
    </option>
  </listbox>
</selectlist>

<div id="support">
  <p>Your browser doesn't support <code>selectlist</code> elements yet.</p>
</div>
!

CSS 

              
                @import "https://unpkg.com/open-props" layer(design.system);
@import "https://unpkg.com/open-props/normalize.min.css" layer(demo.support);
@import "https://unpkg.com/open-props/buttons.min.css" layer(demo.support);

@layer demo {
  selectlist {
    font-size: var(--font-size-1);
    
    > button {
      --_text: var(--text-1);
      
      &:has(selectedoption) {
        align-items: start;
        min-inline-size: 20ch;
        flex-direction: column;
      }
      
      > div {
        inline-size: 100%;
        display: flex;
        justify-content: space-between;
        gap: var(--size-3);
      }
      
      & > small {
        color: var(--text-2);
      }
      
      & svg {
        inline-size: 2ch;
        transition: transform .3s var(--ease-elastic-out-2);
      }
    }
    
    &.primary > button {
      --_bg: var(--link);
      --_border: none;
      --_text: var(--surface-1);
      --_ink-shadow: none;
    }
    
    &:open > button svg {
      transform: rotate(.5turn);
    }
    
    > listbox {
      min-inline-size: calc(anchor-size(self-inline) + 20px);
      margin-block-start: var(--size-2);
      background: var(--surface-2);
/*       box-shadow: var(--shadow-5); */
      border-radius: var(--radius-2);
      padding: 0;
      scroll-behavior: smooth;
      
      opacity: 0;
      clip-path: inset(0 0 100% 0);
      clip-path: circle(0% at top left);
/*       clip-path: polygon(0 0, 50% 50%, 100% 100%, 50% 50%); */
      transition: 
        opacity .1s ease,
        clip-path 0s var(--ease-out-3) .1s, 
        display .1s allow-discrete, 
        overlay .1s allow-discrete
      ;
      
      @media (prefers-color-scheme: light) {
        background: white;
        border: 1px solid var(--surface-2);
      }
      
      &:popover-open {
        transition-duration: 0s, .4s, .4s, .4s;
        opacity: 1;
        clip-path: inset(0 0 0 0);
        clip-path: circle(150% at top left);
        
        @starting-style {
          opacity: 0;
          clip-path: inset(0 0 100% 0);
          clip-path: circle(0% at top left);
        }
      }
      
      & hr {
        margin-block: var(--size-2);
      }
      
      & label {
        display: block;
        position: sticky;
        top: 0;
        z-index: 1;
        background: var(--surface-3);
        font-size: var(--font-size-0);
        color: var(--text-2);
        font-weight: var(--font-weight-7);
        padding-block: var(--size-1);
        padding-inline: var(--size-3);
      }
      
      & option {
        display: flex;
        justify-content: space-between;
        align-items: center;
        gap: var(--size-3);
        padding-block: var(--size-2);
        padding-inline: var(--size-3);
        margin-block: 1px;
        font-size: var(--font-size-1);
        
        cursor: pointer;
        outline-offset: -1px;
        
        &:focus-visible {
          outline-offset: -1px;
        }
        
        &:is(:focus, :hover) {
/*           background: AccentColor;
          color: AccentColorText; */
          background: oklch(from var(--link) l c h / 25%);
          color: inherit;
        }
        
        &:is(:checked) {
          background: var(--link);
          color: var(--surface-1);
          font-weight: var(--font-weight-8);
          
          &::after {
            content: "";
            block-size: var(--size-4);
            aspect-ratio: 1;
            background: url(https://api.iconify.design/ic:round-check.svg);
            background-size: contain;
          }
        }
      }
    }
  }
}

@layer demo.support {
  body {
    display: grid;
    place-items: start;
    place-content: center;
    padding: var(--size-5);
    gap: var(--size-5);
  }
  
  .custom-option {
    display: flex;
    gap: var(--size-3);
    align-items: center;
    justify-content: space-between;
    
    listbox &:has(.description) {
      display: grid;
      justify-items: start;
      gap: var(--size-2);
      padding-block: var(--size-2);
      text-shadow: none;
      
      & .description {
        color: var(--text-2);
      }
    }
    
    option:is(:checked) & .description {
      font-weight: normal;
      color: var(--surface-2);
    }
    
    selectedoption & .description {
      display: none;
    }
  }
  
  .indicator {
    display: inline-block;
    block-size: var(--size-2);
    inline-size: var(--size-2);
    border-radius: var(--radius-round);
    background: var(--gray-5);
    
    &.success {
      background: var(--green-5);
    }
    
    &.danger {
      background: var(--red-5);
    }
  }
  
  .option-text {
    flex: 2;
  }
  
  .avatar {
    border-radius: var(--radius-round);
    overflow: hidden;
  }
}

#support {
  position: fixed;
  inset: 0;
  background: var(--surface-1);
  display: grid;
  place-content: center;
  
  .supports-selectlist & {
    display: none;
  }
}
!

JS 

              
                document.body.classList.toggle('supports-selectlist', "HTMLSelectListElement" in window)

colorspace.oninput = e => {
  console.log(colorspace.value)
}
!
999px

Console