CSS Tooltip Without JavaScript

<p class="ui-tip-text">
  Before you launch in China, check your
  <span class="ui-tip">
    <button class="ui-tip__term" type="button" aria-describedby="ui-tip-icp">ICP filing</button>
    <span class="ui-tip__bubble" role="tooltip" id="ui-tip-icp">A registration with China's Ministry of Industry and Information Technology. Websites hosted in mainland China need one.</span>
  </span>
  and research demand in
  <span class="ui-tip">
    <button class="ui-tip__term" type="button" aria-describedby="ui-tip-index">Baidu Index</button>
    <span class="ui-tip__bubble" role="tooltip" id="ui-tip-index">Baidu's search trend tool (百度指数) — like Google Trends, for searches on Baidu.</span>
  </span>.
</p>

/* Tooltip — no JavaScript.
   The hint is real text in the HTML, linked to its trigger with
   aria-describedby. It shows on hover AND on keyboard focus. */
.ui-tip-text {
  margin: 0;
  font: 400 1.0625rem/1.9 system-ui, -apple-system, "Segoe UI", sans-serif;
  color: #1b1f3b;
}
.ui-tip {
  position: relative;
  display: inline-block;
}

/* The term: a real button, so keyboard users can reach it */
.ui-tip__term {
  padding: 0;
  border: 0;
  border-bottom: 2px dotted #4f46e5;
  background: none;
  font: inherit;
  font-weight: 600;
  color: inherit;
  cursor: help;
}
.ui-tip__term:focus-visible {
  outline: 2px solid #4f46e5;
  outline-offset: 3px;
  border-radius: 3px;
}

/* The bubble: hidden until hover or focus */
.ui-tip__bubble {
  position: absolute;
  bottom: calc(100% + 10px);
  left: 50%;
  z-index: 10;
  width: max-content;
  max-width: min(17rem, 80vw);
  padding: 0.6rem 0.8rem;
  border-radius: 8px;
  background: #1b1f3b;
  color: #fff;
  font-size: 0.875rem;
  font-weight: 400;
  line-height: 1.5;
  text-align: left;
  translate: -50% 0;
  /* display: none while hidden, so off-screen bubbles never
     make the page scroll sideways on phones */
  display: none;
  opacity: 0;
  transition: opacity 0.15s, display 0.15s allow-discrete;
}
/* Small arrow */
.ui-tip__bubble::after {
  content: "";
  position: absolute;
  top: 100%;
  left: 50%;
  translate: -50% 0;
  border: 6px solid transparent;
  border-top-color: #1b1f3b;
}
/* Invisible bridge: the mouse can move onto the bubble without it closing */
.ui-tip__bubble::before {
  content: "";
  position: absolute;
  top: 100%;
  left: 0;
  right: 0;
  height: 12px;
}

.ui-tip:hover .ui-tip__bubble,
.ui-tip:focus-within .ui-tip__bubble {
  display: block;
  opacity: 1;
}
/* Fade in (browsers without @starting-style just show it) */
@starting-style {
  .ui-tip:hover .ui-tip__bubble,
  .ui-tip:focus-within .ui-tip__bubble {
    opacity: 0;
  }
}

<p class="ui-tip-text">
  We also help Western brands with Chinese GEO
  <button class="ui-toggletip__button" type="button" popovertarget="ui-geo-tip"
          style="anchor-name: --ui-geo" aria-label="What is Chinese GEO?">?</button>
</p>

<div class="ui-toggletip" id="ui-geo-tip" popover style="position-anchor: --ui-geo">
  <p><strong>GEO</strong> (generative engine optimisation) makes your content show up in the answers of AI assistants — in China, the assistants Chinese users ask instead of a search engine.</p>
  <p><a href="/services/geo/china/">How we do Chinese GEO</a></p>
</div>

/* Toggletip — no JavaScript.
   Click (or Enter) opens it, Esc or a click outside closes it.
   It can contain links. Anchor positioning puts it next to its button
   and flips it below when there is no room above. */
.ui-toggletip__button {
  display: inline-grid;
  place-items: center;
  width: 1.5rem;
  height: 1.5rem;
  margin-left: 0.2rem;
  padding: 0;
  border: 0;
  border-radius: 50%;
  background: #4f46e5;
  color: #fff;
  font: 700 0.875rem/1 system-ui, -apple-system, "Segoe UI", sans-serif;
  vertical-align: 0.1em;
  cursor: pointer;
}
.ui-toggletip__button:focus-visible,
.ui-toggletip a:focus-visible {
  outline: 2px solid #4f46e5;
  outline-offset: 3px;
}

.ui-toggletip {
  margin: auto; /* fallback: centred on screen — survives a "* { margin: 0 }" reset */
  width: min(19rem, calc(100vw - 2rem));
  padding: 0.9rem 1rem;
  border: 0;
  border-radius: 10px;
  background: #1b1f3b;
  color: #fff;
  box-shadow: 0 14px 34px rgb(10 12 25 / 0.3);
  font: 400 0.9rem/1.55 system-ui, -apple-system, "Segoe UI", sans-serif;
}
.ui-toggletip p {
  margin: 0 0 0.5rem;
}
.ui-toggletip p:last-child {
  margin-bottom: 0;
}
.ui-toggletip a {
  color: #b9b5ff;
  font-weight: 600;
}

/* Next to its button, wherever anchor positioning is supported */
@supports (anchor-name: --a) {
  .ui-toggletip {
    inset: auto;
    margin: 0 0 10px;
    position-area: top;
    position-try-fallbacks: flip-block;
  }
}