Anchor-Positioned Popover

<!-- Anchor-Positioned Popover — the native Popover API (popover /
     popovertarget) plus CSS anchor positioning (anchor-name /
     position-anchor) place and toggle the panel. Esc and outside-click
     close it automatically. Zero JS. -->
<div class="nuda-co2-anchor-popover">
  <button class="nuda-co2-anchor-popover__trigger" type="button"
          popovertarget="co2-ap-tip" popovertargetaction="toggle">
    Show info
  </button>
  <div class="nuda-co2-anchor-popover__tip" id="co2-ap-tip" popover="auto">
    Positioned automatically next to its trigger using native CSS anchor positioning — zero JS.
  </div>
</div>

.nuda-co2-anchor-popover {
  --nuda-co2-ap-accent: #e4ff54;
  position: relative;
  display: inline-flex;
  font-family: inherit;
}

.nuda-co2-anchor-popover__trigger {
  anchor-name: --nuda-co2-ap-anchor;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 44px;
  padding: .5rem 1.1rem;
  font-size: .78rem;
  font-weight: 600;
  color: #fff;
  background: rgba(255,255,255,.06);
  border: 1px solid rgba(255,255,255,.14);
  border-radius: 8px;
  cursor: pointer;
  transition: background .2s ease;
}

.nuda-co2-anchor-popover__trigger:hover {
  background: rgba(255,255,255,.1);
}

.nuda-co2-anchor-popover__trigger:focus-visible {
  outline: 2px solid var(--nuda-co2-ap-accent);
  outline-offset: 2px;
}

.nuda-co2-anchor-popover__tip {
  position: fixed;
  position-anchor: --nuda-co2-ap-anchor;
  position-area: top;
  inset-area: top;
  margin: 0 0 10px;
  max-width: 200px;
  padding: .6rem .75rem;
  font-size: .72rem;
  line-height: 1.45;
  color: #09090b;
  background: var(--nuda-co2-ap-accent);
  border: none;
  border-radius: 9px;
  box-shadow: 0 16px 36px rgba(0,0,0,.45);
}

.nuda-co2-anchor-popover__tip:popover-open {
  animation: _nuda-co2anchorpopoverpop .18s ease-out;
}

@keyframes _nuda-co2anchorpopoverpop {
  from {
    opacity: 0;
    transform: translateY(4px) scale(.96);
  }
  to {
    opacity: 1;
    transform: translateY(0) scale(1);
  }
}

@media (prefers-reduced-motion:reduce) {
  .nuda-co2-anchor-popover__trigger {
    transition: none;
  }
  .nuda-co2-anchor-popover__tip:popover-open {
    animation: none;
  }
}