Cross-dissolve

<div class="frame">
  <div class="shot day"><i class="sun"></i><i class="ridge"></i><b>DAY</b></div>
  <div class="shot night"><i class="moon"></i><i class="ridge"></i><b>NIGHT</b></div>
  <div class="caption">A <span>+</span> B</div>
  <div class="track"><i></i></div>
</div>

.frame{position:absolute;inset:0;overflow:hidden;background:var(--bg)}
.shot{position:absolute;inset:0 0 17%;overflow:hidden}
.day{background:linear-gradient(145deg,#edc980,var(--accent-3));animation:day 3.6s ease-in-out infinite}
.night{background:linear-gradient(145deg,var(--accent-2),#19263b);opacity:0;animation:night 3.6s ease-in-out infinite}
.sun,.moon{position:absolute;left:18%;top:19%;width:clamp(34px,18vmin,100px);aspect-ratio:1;border-radius:50%;background:#fff1b1}
.moon{left:auto;right:18%;background:var(--surface);box-shadow:0 0 0 8px rgba(255,255,255,.12)}
.ridge{position:absolute;left:-5%;right:-5%;bottom:-27%;height:58%;border-radius:50% 50% 0 0;background:var(--fg);opacity:.68;transform:rotate(-5deg)}
.night .ridge{background:#101923;opacity:1;transform:rotate(5deg)}
.shot b{position:absolute;left:7%;bottom:9%;font-size:clamp(10px,3vmin,18px);letter-spacing:.22em;color:var(--surface)}
.caption{position:absolute;right:6%;top:6%;font:700 clamp(10px,2.7vmin,15px) ui-monospace,monospace;color:var(--fg)}
.caption span{color:var(--accent)}
.track{position:absolute;left:7%;right:7%;bottom:7%;height:2px;background:var(--line)}
.track i{display:block;width:50%;height:100%;background:var(--accent);animation:play 3.6s linear infinite}
@keyframes day{0%,32%{opacity:1}56%,100%{opacity:0}}
@keyframes night{0%,32%{opacity:0}56%,100%{opacity:1}}
@keyframes play{from{transform:translateX(0)}to{transform:translateX(100%)}}