mcattx/bubbles.css

## bubbles.css
.bg-bubbles {
    position: absolute;
    // 使气泡背景充满整个屏幕；
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    // 如果元素内容超出给定的宽度和高度，overflow 属性可以确定是否显示滚动条等行为；
    overflow: hidden;
    li {
      position: absolute;
      // bottom 的设置是为了营造出气泡从页面底部冒出的效果；
      bottom: -160px;
      // 默认的气泡大小；
      width: 40px;
      height: 40px;
      background-color: rgba(255, 255, 255, 0.15);
      list-style: none;
      // 使用自定义动画使气泡渐现、上升和翻滚；
      animation: square 15s infinite;
      transition-timing-function: linear;
      // 分别设置每个气泡不同的位置、大小、透明度和速度，以显得有层次感；
      &:nth-child(1) {
        left: 10%;
      }
      &:nth-child(2) {
        left: 20%;
        width: 90px;
        height: 90px;
        animation-delay: 2s;
        animation-duration: 7s;
      }
      &:nth-child(3) {
        left: 25%;
        animation-delay: 4s;
      }
      &:nth-child(4) {
        left: 40%;
        width: 60px;
        height: 60px;
        animation-duration: 8s;
        background-color: rgba(255, 255, 255, 0.3);
      }
      &:nth-child(5) {
        left: 70%;
      }
      &:nth-child(6) {
        left: 80%;
        width: 120px;
        height: 120px;
        animation-delay: 3s;
        background-color: rgba(255, 255, 255, 0.2);
      }
      &:nth-child(7) {
        left: 32%;
        width: 160px;
        height: 160px;
        animation-delay: 2s;
      }
      &:nth-child(8) {
        left: 55%;
        width: 20px;
        height: 20px;
        animation-delay: 4s;
        animation-duration: 15s;
      }
      &:nth-child(9) {
        left: 25%;
        width: 10px;
        height: 10px;
        animation-delay: 2s;
        animation-duration: 12s;
        background-color: rgba(255, 255, 255, 0.3);
      }
      &:nth-child(10) {
        left: 85%;
        width: 160px;
        height: 160px;
        animation-delay: 5s;
      }
    }
    // 自定义 square 动画；
    @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);
      }
    }
  }

body {
  width: 100%;
  height: 1000px;
  background: linear-gradient(to bottom right, #50A3A2, #53E3A6);
}


## bubbles.html
<body>
  <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>
</body>
	.bg-bubbles {
	position: absolute;
	// 使气泡背景充满整个屏幕；
	top: 0;
	left: 0;
	width: 100%;
	height: 100%;
	// 如果元素内容超出给定的宽度和高度，overflow 属性可以确定是否显示滚动条等行为；
	overflow: hidden;
	li {
	position: absolute;
	// bottom 的设置是为了营造出气泡从页面底部冒出的效果；
	bottom: -160px;
	// 默认的气泡大小；
	width: 40px;
	height: 40px;
	background-color: rgba(255, 255, 255, 0.15);
	list-style: none;
	// 使用自定义动画使气泡渐现、上升和翻滚；
	animation: square 15s infinite;
	transition-timing-function: linear;
	// 分别设置每个气泡不同的位置、大小、透明度和速度，以显得有层次感；
	&:nth-child(1) {
	left: 10%;
	}
	&:nth-child(2) {
	left: 20%;
	width: 90px;
	height: 90px;
	animation-delay: 2s;
	animation-duration: 7s;
	}
	&:nth-child(3) {
	left: 25%;
	animation-delay: 4s;
	}
	&:nth-child(4) {
	left: 40%;
	width: 60px;
	height: 60px;
	animation-duration: 8s;
	background-color: rgba(255, 255, 255, 0.3);
	}
	&:nth-child(5) {
	left: 70%;
	}
	&:nth-child(6) {
	left: 80%;
	width: 120px;
	height: 120px;
	animation-delay: 3s;
	background-color: rgba(255, 255, 255, 0.2);
	}
	&:nth-child(7) {
	left: 32%;
	width: 160px;
	height: 160px;
	animation-delay: 2s;
	}
	&:nth-child(8) {
	left: 55%;
	width: 20px;
	height: 20px;
	animation-delay: 4s;
	animation-duration: 15s;
	}
	&:nth-child(9) {
	left: 25%;
	width: 10px;
	height: 10px;
	animation-delay: 2s;
	animation-duration: 12s;
	background-color: rgba(255, 255, 255, 0.3);
	}
	&:nth-child(10) {
	left: 85%;
	width: 160px;
	height: 160px;
	animation-delay: 5s;
	}
	}
	// 自定义 square 动画；
	@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);
	}
	}
	}

	body {
	width: 100%;
	height: 1000px;
	background: linear-gradient(to bottom right, #50A3A2, #53E3A6);
	}
	<body>
	<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>
	</body>