highlight.css

/* Temporary highlight applied by highlight.js to the target message of a
 * message link: hold a translucent yellow for a few seconds, then fade out.
 *
 * The animation runs on both the tagged list item and the message background
 * inside it — the background can mount after the item is tagged, and
 * depending on the theme either can be the opaque surface that would cover
 * the other's paint. A translucent tone is used so it reads on both light and
 * dark themes. Animated properties win over Slack's own static background
 * declarations while the animation runs — and only while it runs (no fill
 * mode), so Slack's background comes back untouched when the fade ends and no
 * !important is needed. */
@keyframes slack-open-browser-highlight-fade {
  0%,
  55% {
    background-color: rgba(242, 199, 68, 0.1);
  }
  100% {
    background-color: transparent;
  }
}

.slack-open-browser-highlight,
.slack-open-browser-highlight .c-message_kit__background {
  /* Duration must stay in sync with FADE_MS in highlight.js. */
  animation: slack-open-browser-highlight-fade 6s ease-out;
  /* highlight.js sets the custom property to a negative offset when it tags a
   * node after the fade already started (a recycled or class-stripped row),
   * so the animation resumes mid-fade instead of restarting. Declared after
   * the shorthand, which would otherwise reset the delay; inherits to the
   * descendant. */
  animation-delay: var(--slack-open-browser-elapsed, 0ms);
}