SOURCE

  <ul class="bg-bubbles">
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
</ul>

.bg-bubbles {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  overflow: hidden; 
  background: linear-gradient(to bottom right, #50A3A2, #53E3A6);

}
  .bg-bubbles li {
    position: absolute;
    bottom: -160px;
    width: 40px;
    height: 40px;
    background-color: rgba(255, 255, 255, 0.15);
    list-style: none;
    animation: square 15s infinite;
    transition-timing-function: linear;
    border-radius: 50%;
}
    .bg-bubbles li:nth-child(1) {
      left: 10%; }
    .bg-bubbles li:nth-child(2) {
      left: 20%;
      width: 90px;
      height: 90px;
      animation-delay: 2s;
      animation-duration: 7s; }
    .bg-bubbles li:nth-child(3) {
      left: 25%;
      animation-delay: 4s; }
    .bg-bubbles li:nth-child(4) {
      left: 40%;
      width: 60px;
      height: 60px;
      animation-duration: 8s;
      background-color: rgba(255, 255, 255, 0.3); }
    .bg-bubbles li:nth-child(5) {
      left: 70%; }
    .bg-bubbles li:nth-child(6) {
      left: 80%;
      width: 120px;
      height: 120px;
      animation-delay: 3s;
      background-color: rgba(255, 255, 255, 0.2); }
    .bg-bubbles li:nth-child(7) {
      left: 32%;
      width: 160px;
      height: 160px;
      animation-delay: 2s; }
    .bg-bubbles li:nth-child(8) {
      left: 55%;
      width: 20px;
      height: 20px;
      animation-delay: 4s;
      animation-duration: 15s; }
    .bg-bubbles li:nth-child(9) {
      left: 25%;
      width: 10px;
      height: 10px;
      animation-delay: 2s;
      animation-duration: 12s;
      background-color: rgba(255, 255, 255, 0.3); }
    .bg-bubbles li:nth-child(10) {
      left: 85%;
      width: 160px;
      height: 160px;
      animation-delay: 5s; }
@keyframes square {
  0% {
    opacity: 0.5;
    transform: translateY(0px) rotate(45deg); }
  25% {
    opacity: 0.75;
    transform: translateY(-400px) rotate(90deg); }
  50% {
    opacity: 1;
    transform: translateY(-600px) rotate(135deg); }
  100% {
    opacity: 0;
    transform: translateY(-1000px) rotate(180deg); } }