CSSのみで円形が上へゆっくりと登っていく背景アニメーションをつくりました

   <div class="context">
        <h1>Pure Css Animated Background</h1>
    </div>

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

@import url('https://fonts.googleapis.com/css?family=Exo:400,700');

*{
    margin: 0px;
    padding: 0px;
}

body{
    font-family: 'Exo', sans-serif;
}


.context {
    width: 100%;
    position: absolute;
    top:50vh;
    
}

.context h1{
    text-align: center;
    color: #fff;
    font-size: 50px;
}

.bubbles {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    overflow: hidden;
    z-index: -1;
    background: #111; /* 背景色（お好みで変更可能） */
}

.bubbles li {
    position: absolute;
    display: block;
    list-style: none;
    width: 15px;
    height: 15px;
    background: rgba(255, 255, 255, 0.1);
    border-radius: 50%;
    animation: bubbleAnimate 20s linear infinite;
    bottom: -50px;
    opacity: 0.8;
}

/* バブルごとのスタイル調整 */
.bubbles li:nth-child(1) {
    left: 20%;
    width: 20px;
    height: 20px;
    animation-duration: 18s;
    animation-delay: 2s;
}

.bubbles li:nth-child(2) {
    left: 40%;
    width: 25px;
    height: 25px;
    animation-duration: 22s;
    animation-delay: 4s;
}

.bubbles li:nth-child(3) {
    left: 60%;
    width: 10px;
    height: 10px;
    animation-duration: 25s;
    animation-delay: 1s;
}

.bubbles li:nth-child(4) {
    left: 80%;
    width: 15px;
    height: 15px;
    animation-duration: 20s;
    animation-delay: 3s;
}

.bubbles li:nth-child(5) {
    left: 30%;
    width: 30px;
    height: 30px;
    animation-duration: 30s;
    animation-delay: 6s;
}

.bubbles li:nth-child(6) {
    left: 50%;
    width: 10px;
    height: 10px;
    animation-duration: 24s;
    animation-delay: 8s;
}

.bubbles li:nth-child(7) {
    left: 70%;
    width: 40px;
    height: 40px;
    animation-duration: 28s;
    animation-delay: 10s;
}

.bubbles li:nth-child(8) {
    left: 10%;
    width: 15px;
    height: 15px;
    animation-duration: 26s;
    animation-delay: 5s;
}

.bubbles li:nth-child(9) {
    left: 90%;
    width: 20px;
    height: 20px;
    animation-duration: 30s;
    animation-delay: 3s;
}

.bubbles li:nth-child(10) {
    left: 50%;
    width: 30px;
    height: 30px;
    animation-duration: 32s;
    animation-delay: 7s;
}

/* アニメーション */
@keyframes bubbleAnimate {
    0% {
        transform: translateY(0) scale(1);
        opacity: 0.5;
    }
    50% {
        opacity: 1;
    }
    100% {
        transform: translateY(-100vh) scale(1.2);
        opacity: 0;
    }
}