<div id="wrap">

  <!-- section -->
  <div class="section">
    <div class="center">

      <div class="block">
        <div class="block__content"></div>
      </div>

    </div>
  </div>
  <!--/section -->

</div>

html,
body {
  margin: 0;
  padding: 0;
  font-family: sans-serif;
  font-size: 16px;
  line-height: 1.4;
  color: #555;
  background: #fff;
}

* {
  box-sizing: border-box;
}

#wrap {
  position: relative;
}

.section {
  padding: 40px;
}

.block {
  width: 300px;
  height: 200px;
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
  position: relative;
  
  &::before {
    content: '';
    display: block;
    position: absolute;
    width: 200%;
    height: 200%;
    top: -50%;
    left: -50%;
    background-image: linear-gradient(to right, #ccc 50%, #000 50%);
    animation: rotate 2s linear infinite;
    
    @keyframes rotate {
      to {
        transform: rotate(360deg);
      }
    }
  }
  
  &::after {
    content: '';
    display: block;
    position: absolute;
    z-index: 1;
    top: 2px;
    right: 2px;
    bottom: 2px;
    left: 2px;
    background: #fff;
  }
  
  &__content {
    position: relative;
    z-index: 2;
    width: 240px;
    height: 140px;
    background: #000;
  }
}