Button-hover 04

HTML

<div class="container">
  <button class="btn">Button</button>
</div>  

CSS

body {
  margin: 0;
  padding: 0;
  background: #34495e;
  font-family: sans-serif;
}
.container {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}
.btn {
  text-transform: uppercase;
  text-decoration: none;
  letter-spacing: 2px;
  font-size: 24px;
  color: #ecf0f1;
  padding: 20px 44px;
  outline: none;
  border: none;
  border-radius: 40px;
  text-align: center;
  box-sizing: border-box;
  box-shadow: 0 2px 5px rgba(0, 0, 0, .3);
  background: linear-gradient(90deg, #95a5a6, #2ecc71, #f1c40f, #d35400, #95a5a6);
  background-size: 200%;
}
.btn:hover {
  animation: gradient 4s linear infinite;
}
@keyframes gradient {
  0% {
    background-position-x: 0;
  }
  100% {
    background-position-x: 200%;
  }
}
.btn::before {
  content: '';
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 130px;
  height: 40px;
  padding: 20px 44px;
  border-radius: 40px;
  z-index: -1;
  background: linear-gradient(90deg, #95a5a6, #2ecc71, #f1c40f, #d35400, #95a5a6);
  background-size: 200%;
  opacity: 0;
  transition: .5s;
}
.btn:hover::before {
  filter: blur(20px);
  opacity: 1;
  animation: gradient 4s linear infinite;
}

Demo

Description
background-size: 200%;: 尺寸放大，方便动画渐变

这两个我一般都是一起用，清除原有的边框轮廓

outline: none;
border: none;

filter: blur(20px): 加上高斯模糊，因为背景虚化是用::before 定位到的，所以这里使用模糊使得背景自然一些