Fold In

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Fold In: the element unfolds from a crease across its middle. Both
    // halves start almost edge-on, folded away from you along the crease, and
    // open out flat like a greetings card, brightening as they face you.
    // Usage: <div data-fold-in>…</div>
    // While it unfolds, a copy of the element (aria-hidden) sits on top of it
    // to draw the second half, so the element's parent should be its
    // positioned container (the copy is placed with offsetLeft/offsetTop).
    // Optional: data-axis ("x", default, a horizontal crease that opens up
    // and down; "y" a vertical crease that opens sideways; give both, "x y",
    // and each loop uses the next), data-angle (how far folded it starts, in
    // degrees, default 85), data-perspective (px, default 700),
    // data-duration (ms, default 1000), data-delay (ms before the first
    // entrance), data-loop (hold, fade away and unfold again), data-hold (ms
    // to stay when looping, default 1400).
    var DEFAULTS = { axis: 'x', angle: 85, perspective: 700, duration: 1000, delay: 0, hold: 1400 };

    function reduced() {
      return window.matchMedia('(prefers-reduced-motion: reduce)').matches;
    }

    // A copy of the element laid exactly over it, to be the second half.
    function makeCopy(el) {
      var c = el.cloneNode(true);
      c.removeAttribute('data-fold-in');
      c.removeAttribute('id');
      c.querySelectorAll('[id]').forEach(function (n) { n.removeAttribute('id'); });
      c.setAttribute('aria-hidden', 'true');
      c.style.position = 'absolute';
      c.style.left = el.offsetLeft + 'px';
      c.style.top = el.offsetTop + 'px';
      c.style.width = el.offsetWidth + 'px';
      c.style.height = el.offsetHeight + 'px';
      c.style.margin = '0';
      c.style.boxSizing = 'border-box';
      c.style.pointerEvents = 'none';
      el.parentNode.insertBefore(c, el.nextSibling);
      return c;
    }

    function enter(el, axis, done) {
      var d = el.dataset;
      var t = +d.duration || DEFAULTS.duration;
      var a = d.angle === undefined ? DEFAULTS.angle : +d.angle || 0;
      var p = (+d.perspective || DEFAULTS.perspective) + 'px';
      var rot = axis === 'y' ? 'rotateY' : 'rotateX';
      var copy = makeCopy(el);
      // The element draws the first half (top or left), the copy the second.
      // Signs tip both outer edges away from the viewer, so the crease sits nearest.
      var first = axis === 'y' ? -a : a;
      el.style.clipPath = axis === 'y' ? 'inset(0 50% 0 0)' : 'inset(0 0 50% 0)';
      copy.style.clipPath = axis === 'y' ? 'inset(0 0 0 50%)' : 'inset(50% 0 0 0)';
      el.style.transformOrigin = copy.style.transformOrigin = '50% 50%';
      var halves = [[el, first], [copy, -first]];
      var tl = anime.createTimeline({
        onComplete: function () {
          copy.remove();
          el.style.clipPath = '';
          el.style.filter = '';
          done();
        }
      });
      halves.forEach(function (h) {
        var start = { perspective: p, opacity: 0, filter: 'brightness(0.6)' };
        start[rot] = h[1];
        anime.utils.set(h[0], start);
        var props = {
          opacity: { to: 1, duration: t * 0.2, ease: 'linear' },
          filter: { to: 'brightness(1)', duration: t * 0.72, ease: 'out(2)' }
        };
        // Open flat, just past it, and settle.
        props[rot] = [
          { to: -h[1] * 0.05, duration: t * 0.72, ease: 'out(2)' },
          { to: 0, duration: t * 0.3, ease: 'inOut(2)' }
        ];
        tl.add(h[0], props, 0);
      });
    }

    function cycle(el) {
      var d = el.dataset;
      var s = el._fdi;
      var axes = (d.axis || DEFAULTS.axis).split(/[\s,]+/);
      enter(el, axes[s.n++ % axes.length], function () {
        if (d.loop === undefined || d.loop === 'false') return;
        setTimeout(function () {
          anime.animate(el, {
            opacity: 0,
            duration: 400,
            ease: 'out(2)',
            onComplete: function () {
              // Wait until it's back on screen before the next entrance.
              setTimeout(function () {
                if (s.visible) cycle(el);
                else s.waiting = true;
              }, 500);
            }
          });
        }, +d.hold || DEFAULTS.hold);
      });
    }

    function init() {
      var io = new IntersectionObserver(function (entries) {
        entries.forEach(function (e) {
          var s = e.target._fdi;
          s.visible = e.isIntersecting;
          if (!e.isIntersecting) return;
          if (!s.started) {
            s.started = true;
            setTimeout(function () { cycle(e.target); }, +e.target.dataset.delay || DEFAULTS.delay);
          } else if (s.waiting) {
            s.waiting = false;
            cycle(e.target);
          }
        });
      }, { threshold: 0.2 });
      document.querySelectorAll('[data-fold-in]').forEach(function (el) {
        if (el._fdi) return;
        el._fdi = { n: 0 };
        if (reduced()) return;
        anime.utils.set(el, { opacity: 0 });
        io.observe(el);
      });
    }

    if (document.readyState === 'loading') document.addEventListener('DOMContentLoaded', init);
    else init();
  })();
</script>

<div style="position: relative">
  <div class="card" data-fold-in>You're invited</div>
</div>

<!-- or a vertical crease that opens sideways: -->
<div class="card" data-fold-in data-axis="y">You're invited</div>