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HTML 

              
                <!-- Demo changing frequently atm -->
<select>
  <button>
    <div>
      <selectedcontent></selectedcontent>
      <svg width="24" height="24" viewBox="0 0 24 24">
        <path fill="currentColor" d="m7 10l5 5l5-5z"/>
      </svg>
    </div>
  </button>
  <div>
    <option value="on">
      <!-- custom so it's fully consumed for selectedcontent -->
      <div class="custom-option">
        <span class="indicator success"></span>
        <span class="option-text">On</span>
      </div>
    </option>
    <option value="off">
      <div class="custom-option">
        <span class="indicator danger"></span>
        <span class="option-text">Off</span>
      </div>
    </option>
    <option value="disable">
      <div class="custom-option">
        <span class="indicator disable"></span>
        <span class="option-text">Disable</span>
      </div>
    </option>
  </div>
</select>

<select> 
  <button>
    <div>
      <selectedcontent></selectedcontent>
      <svg width="24" height="24" viewBox="0 0 24 24">
        <path fill="currentColor" d="m7 10l5 5l5-5z"/>
      </svg>
    </div>
  </button>
  <div>
    <option value="Una Kravets">
      <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 value="Bramus Van Damme"> 
      <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 value="Adam Argyle">
      <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>
  </div>
</select>

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

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

<div id="support">
  <p>Your browser doesn't support <code>customizable select</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);

/*  
  TODO:
    - large excellent mobile design
    - excellent tablet design
    - anchor position keywords not math + most-height feature
*/


select {
  /* opt into customizing select */
  &, &::picker(select) {
    appearance: base-select;
  }
  
  font-size: var(--font-size-1);
  
  /* removing open props normalize styles */
  background: none;
  padding: 0;
  
  /* enable transitions in the drop down */
  &::picker(select) {
    transition: 
      opacity .2s ease .2s,
      clip-path .2s var(--ease-out-3), 
      display .2s allow-discrete, 
      overlay .2s allow-discrete
    ;
  }
  
  &::picker-icon {
    display: none;
  }
  
  /* set the off stage styles */
  &:not(:open)::picker(select) {
    opacity: 0;
    clip-path: inset(0 0 100% 0);
    clip-path: circle(0% at top left);
  }
  
  /* set the on stage styles */
  &:open::picker(select) {
    transition-duration: 0s, .4s, .4s, .4s;
    transition-delay: 0s;
    opacity: 1;
    clip-path: inset(0 0 0 0);
    clip-path: circle(150% at top left);
    
    /* set where to enter stage from */
    @starting-style {
      opacity: 0;
      clip-path: inset(0 0 100% 0);
      clip-path: circle(0% at top left);
    }
  }
  
  /* transition the selected option changing */
  selectedcontent > * {
    transition: 
      transform 1s var(--ease-spring-4),
      display 1s allow-discrete, 
      opacity 1s;
    
    @starting-style {
      opacity: 0;
      transform: translateY(10px);
    }
    opacity: 1;
  }
  
  /* customize the invoking button */
  > button {
    --_text: var(--text-1);
  
    &:focus-visible {
      outline-offset: -3px;
    }

    &:has(selectedcontent) {
      align-items: start;
      min-inline-size: 20ch;
      flex-direction: column;
    }
    
    &.primary {
      --_bg: var(--link);
      --_border: none;
      --_text: var(--surface-1);
      --_ink-shadow: none;
    }

    > 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);
    }
  }
  
  /* reset some picker styles */
  &::picker(select) {
    background: var(--surface-2);
    border-radius: var(--radius-2);
    padding: 0;
    border: none;
    margin-block-start: 5px;
    box-shadow: var(--shadow-5);
  }
  
  /* customize the picker contents */
  > div {
    inset-block-start: calc(anchor(bottom) + 5px);
    position-try-fallbacks: flip-block;
    min-inline-size: calc(anchor-size(self-inline) + 20px);
    scroll-behavior: smooth;
    
    &.scrollable {
      max-block-size: 20lh;
      inset-block-start: anchor(top);
      inset-inline-start: calc(anchor(right) + 5px);
      position-try-fallbacks: flip-inline;
    }

    @media (prefers-color-scheme: light) {
      background: white;
      border: 1px solid var(--surface-2);
    }

    & 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;
      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: 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);
      }
    }
  }
}

/* utilities to aid in custom select children */
.custom-option {
  display: flex;
  gap: var(--size-3);
  align-items: center;
  justify-content: space-between;

  .primary & {
    display: grid;
    gap: var(--size-1);
  }

  &: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);
  }

  selectedcontent & .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;
}







@layer demo.support {
  body {
    display: grid;
    place-items: start;
    place-content: center;
    padding: var(--size-5);
    gap: var(--size-5);
  }
}

#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', 
  CSS.supports("appearance: base-select")
)

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

Console