interpolate-size is a way to animate height: auto or width. It is only supported in Chrome for now.

A quick example:

@supports (interpolate-size: allow-keywords) {
  :root {
    interpolate-size: allow-keywords;
  }

  details {
    transition: height 0.5s ease;
    height: 2.5rem;

    &[open] {
      height: auto;
      overflow: clip;
    }
  }
}

A more generic one:

:root {
  interpolate-size: allow-keywords;
}

details {
  --calculated-details-padding: var(--details-padding, 1em);

  transition: height var(--details-transition-speed, 150ms) linear;
  height: calc(1lh + (var(--calculated-details-padding) * 2));
  padding: var(--calculated-details-padding);
  background: var(--details-bg, white);
  color: var(--details-color, currentColor);
}

details[open] {
  height: auto;
  overflow: clip;
}