<div class="tooltip">Anchor me!</div>
<a class="anchor">The anchor</a>

@import "https://unpkg.com/open-props/open-props.min.css";
@import "https://unpkg.com/open-props/normalize.min.css";

* {
  box-sizing: border-box;
}

body {
  font-family: "Google Sans", sans-serif;
  display: grid;
  place-items: center;
  min-height: 100vh;
  background: var(--surface-2);
  color: var(--text-1);
}

.containing-block {
  position: relative;
}
.tooltip {
  position: absolute;
  top: calc(var(--y) * 1px);
  left: calc(((var(--width) * 0.5) + var(--x)) * 1px);
  transform: translate(-50%, calc(-100% - 10px));
  box-shadow: var(--shadow-4);
  padding: var(--size-4);
  white-space: nowrap;
  border-radius: var(--radius-2);
  font-weight: var(--font-weight-6);
  background: var(--surface-1);
  border: 1px solid var(--surface-3);
}

.anchor {
  background: var(--blue-4);
  text-align: center;
  font-weight: bold;
  border-radius: var(--radius-2);
  padding: var(--size-2);
}

const setAnchorPosition = (anchored, anchor) => {
  const bounds = anchor.getBoundingClientRect().toJSON();
  for (const [key, value] of Object.entries(bounds)) {
    anchored.style.setProperty(`--${key}`, value);
  }
};

const update = () => {
  setAnchorPosition(
    document.querySelector(".tooltip"),
    document.querySelector(".anchor")
  );
};

window.addEventListener("resize", update);
document.addEventListener("DOMContentLoaded", update);